Add support for X11 primary selection (#6132)

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>`

include/SDL_clipboard.h

@@ -82,6 +82,52 @@ extern DECLSPEC char * SDLCALL SDL_GetClipboardText(void);
  */
 extern DECLSPEC SDL_bool SDLCALL SDL_HasClipboardText(void);
 
+/**
+ * Put UTF-8 text into the primary selection.
+ *
+ * \param text the text to store in the primary selection
+ * \returns 0 on success or a negative error code on failure; call
+ *          SDL_GetError() for more information.
+ *
+ * \since This function is available since SDL 2.25.0.
+ *
+ * \sa SDL_GetPrimarySelectionText
+ * \sa SDL_HasPrimarySelectionText
+ */
+extern DECLSPEC int SDLCALL SDL_SetPrimarySelectionText(const char *text);
+
+/**
+ * Get UTF-8 text from the primary selection, which must be freed with SDL_free().
+ *
+ * This functions returns empty string if there was not enough memory left for
+ * a copy of the primary selection's content.
+ *
+ * \returns the primary selection text on success or an empty string on failure;
+ *          call SDL_GetError() for more information. Caller must call SDL_free()
+ *          on the returned pointer when done with it (even if there was an
+ *          error).
+ *
+ * \since This function is available since SDL 2.25.0.
+ *
+ * \sa SDL_HasPrimarySelectionText
+ * \sa SDL_SetPrimarySelectionText
+ */
+extern DECLSPEC char * SDLCALL SDL_GetPrimarySelectionText(void);
+
+/**
+ * Query whether the primary selection exists and contains a non-empty text
+ * string.
+ *
+ * \returns SDL_TRUE if the primary selection has text, or SDL_FALSE if it does
+ *          not.
+ *
+ * \since This function is available since SDL 2.25.0.
+ *
+ * \sa SDL_GetPrimarySelectionText
+ * \sa SDL_SetPrimarySelectionText
+ */
+extern DECLSPEC SDL_bool SDLCALL SDL_HasPrimarySelectionText(void);
+
 
 /* Ends C function definitions when using C++ */
 #ifdef __cplusplus