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/testoffscreen.c

@@ -23,7 +23,7 @@
 
 static SDL_Renderer *renderer = NULL;
 static SDL_Window *window = NULL;
-static int done = SDL_FALSE;
+static int done = false;
 static int frame_number = 0;
 static int width = 640;
 static int height = 480;
@@ -69,7 +69,7 @@ static void loop(void)
     while (SDL_PollEvent(&event)) {
         switch (event.type) {
         case SDL_EVENT_QUIT:
-            done = SDL_TRUE;
+            done = true;
             break;
         default:
             break;