Removed SDL_bool in favor of plain bool

We require stdbool.h in the build environment, so we might as well use the plain bool type.

If your environment doesn't have stdbool.h, this simple replacement will suffice:
typedef signed char bool;

test/testgpu_simple_clear.c

@@ -41,7 +41,7 @@ SDL_AppResult SDL_AppInit(void **appstate, int argc, char *argv[])
         return SDL_APP_FAILURE;
     }
 
-	gpu_device = SDL_CreateGPUDevice(TESTGPU_SUPPORTED_FORMATS, SDL_TRUE, NULL);
+	gpu_device = SDL_CreateGPUDevice(TESTGPU_SUPPORTED_FORMATS, true, NULL);
 	if (!gpu_device) {
 		SDL_Log("SDL_CreateGPUDevice failed: %s", SDL_GetError());
 		return SDL_APP_FAILURE;