Cleaned up various type conversion issues

This makes sure SDL_PixelFormatEnum flows through the internal code correctly, as well as fixing a number of other minor issues.

test/testgl.c

@@ -310,7 +310,7 @@ int main(int argc, char *argv[])
 
     mode = SDL_GetCurrentDisplayMode(SDL_GetPrimaryDisplay());
     if (mode) {
-        SDL_Log("Screen BPP    : %" SDL_PRIu32 "\n", SDL_BITSPERPIXEL(mode->format));
+        SDL_Log("Screen BPP    : %d\n", SDL_BITSPERPIXEL(mode->format));
     }
 
     LogSwapInterval();