Fixed getting an 8-bit window surface on Apple platforms

The Metal renderer doesn't support XRGB texture formats, so we ended up matching SDL_PIXELFORMAT_INDEX8 for the window surface.

src/video/SDL_video.c

@@ -430,6 +430,7 @@ static bool SDL_CreateWindowTexture(SDL_VideoDevice *_this, SDL_Window *window,
         if (!SDL_ISPIXELFORMAT_FOURCC(texture_format) &&
             !SDL_ISPIXELFORMAT_10BIT(texture_format) &&
             !SDL_ISPIXELFORMAT_FLOAT(texture_format) &&
+            !SDL_ISPIXELFORMAT_INDEXED(texture_format) &&
             transparent == SDL_ISPIXELFORMAT_ALPHA(texture_format)) {
             *format = texture_format;
             break;