Fixes https://github.com/libsdl-org/SDL/issues/6241

Silences clang -Wpragma warnings:

D:/a/SDL/SDL/src/video/windows/SDL_windowstaskdialog.h:21:10: warning: the current #pragma pack alignment value is modified in the included file [-Wpragma-pack]
#include <pshpack1.h>
         ^
D:/a/_temp/msys64/clang64/include/pshpack1.h:7:9: note: previous '#pragma pack' directive that modifies alignment is here
#pragma pack(push,1)
        ^

In file included from D:/a/SDL/SDL/src/video/windows/SDL_windowsmessagebox.c:35:

D:/a/SDL/SDL/src/video/windows/SDL_windowstaskdialog.h:156:10: warning: the current #pragma pack alignment value is modified in the included file [-Wpragma-pack]

#include <poppack.h>

         ^

note: previous '#pragma pack' directive that modifies alignment is here

2 warnings generated.

Closes: https://github.com/libsdl-org/SDL/issues/6240

Unfortunately the only way to detect this is to actually try opening a device, so we wait until the application tries, and then stop enumerating afterwards.

Fixes https://github.com/libsdl-org/SDL/issues/5781

This kicks in if the platform doesn't support vsync directly, or if the present fails for some reason (e.g. minimized on some platforms)

Fixes https://github.com/libsdl-org/SDL/issues/5134

also define them as 1, instead of empty.
Reference issue: https://github.com/libsdl-org/SDL/issues/6239

Fixes redefinition warnings from windows builds.
Reference issue: https://github.com/libsdl-org/SDL/issues/6239

Also did miscellaneous style cleanup for consistency with other code

Fixes https://github.com/libsdl-org/SDL/issues/6231

X11 has a so-called primary selection, which you can use by marking text and middle-clicking elsewhere to copy the marked text.

There are 3 new API functions in `SDL_clipboard.h`, which work exactly like their clipboard equivalents.

## Test Instructions

* Run the tests (just a copy of the clipboard tests): `$ ./test/testautomation --filter Clipboard`
* Build and run this small application:
<details>
```C
#include <SDL.h>
#include <unistd.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>

void print_error(const char *where)
{
	const char *errstr = SDL_GetError();
	if (errstr == NULL || errstr[0] == '\0')
		return;
	fprintf(stderr, "SDL Error after '%s': %s\n", where, errstr);
	SDL_ClearError();
}

int main()
{
	char text_buf[256];

	srand(time(NULL));

	SDL_Init(SDL_INIT_VIDEO);
	print_error("SDL_INIT()");
	SDL_Window *window = SDL_CreateWindow("Primary Selection Test", SDL_WINDOWPOS_UNDEFINED,
			SDL_WINDOWPOS_UNDEFINED, 400, 400, SDL_WINDOW_SHOWN);
	print_error("SDL_CreateWindow()");
	SDL_Renderer *renderer = SDL_CreateRenderer(window, -1, SDL_RENDERER_ACCELERATED);
	print_error("SDL_CreateRenderer()");

	bool quit = false;
	unsigned int do_render = 0;
	while (!quit) {
		SDL_Event event;
		while (SDL_PollEvent(&event)) {
			print_error("SDL_PollEvent()");
			switch (event.type) {
			case SDL_QUIT: {
				quit = true;
				break;
			} case SDL_KEYDOWN: {
				switch (event.key.keysym.sym) {
				case SDLK_ESCAPE:
				case SDLK_q:
					quit = true;
					break;
				case SDLK_c:
					snprintf(text_buf, sizeof(text_buf), "foo%d", rand());
					SDL_SetClipboardText(text_buf);
					print_error("SDL_SetClipboardText()");
					printf("clipboard: set_to=\"%s\"\n", text_buf);
					break;
				case SDLK_v: {
					printf("clipboard: has=%d, ", SDL_HasClipboardText());
					print_error("SDL_HasClipboardText()");
					char *text = SDL_GetClipboardText();
					print_error("SDL_GetClipboardText()");
					printf("text=\"%s\"\n", text);
					SDL_free(text);
					break;
				} case SDLK_d:
					snprintf(text_buf, sizeof(text_buf), "bar%d", rand());
					SDL_SetPrimarySelectionText(text_buf);
					print_error("SDL_SetPrimarySelectionText()");
					printf("primselec: set_to=\"%s\"\n", text_buf);
					break;
				case SDLK_f: {
					printf("primselec: has=%d, ", SDL_HasPrimarySelectionText());
					print_error("SDL_HasPrimarySelectionText()");
					char *text = SDL_GetPrimarySelectionText();
					print_error("SDL_GetPrimarySelectionText()");
					printf("text=\"%s\"\n", text);
					SDL_free(text);
					break;
				} default:
					break;
				}
				break;
			} default: {
				break;
			}}
		}
		// create less noise with WAYLAND_DEBUG=1
		if (do_render == 0) {
			SDL_RenderPresent(renderer);
			print_error("SDL_RenderPresent()");
		}
		do_render += 1;
		usleep(12000);
	}

	SDL_DestroyRenderer(renderer);
	SDL_DestroyWindow(window);
	SDL_Quit();
	print_error("quit");
	return 0;
}
```
</details>

* Use c,v,d,f to get and set the clipboard and primary selection.
* Mark text and middle-click also in other applications.
* For wayland under x:
  * `$ mutter --wayland --no-x11 --nested`
  * `$ XDG_SESSION_TYPE=wayland SDL_VIDEODRIVER=wayland ./<path_to_test_appl_binary>`

Fixes https://github.com/libsdl-org/SDL/issues/6193

Cache all the shaders up front, so we can verify that they won't silently fail at runtime.

If compiling the fragment shaders with the precision hint fails, try again without specifying precision.

Fixes https://github.com/libsdl-org/SDL/issues/6166
Fixes https://github.com/libsdl-org/SDL/issues/6174

Clip rectangle set to int(left+width/2) , int(top+height/2) , int(left+width/2)+1 , int(top+height/2)+1
a 1x1 box
On even-valued resolution, cursor is stable at bottom-right central pixel
On odd-valued resolution, cursor is stable at exact central pixel.
this is the desired behaviour