Only blit neutral surfaces.

On the Mac loaded images are not in the neutral format causing rendering
glitches when using blit_surface using these images. Test and when
needed convert the image before using it.

Fixes bug #22045.

changelog

@@ -42,6 +42,7 @@ Version 1.13.0-dev:
      when wesnoth is running in debug mode (--debug command line flag).
  * Graphics:
    * Smooth unit movement over terrain with elevation (e.g. keeps/bridges)
+   * Fixed bug #22045: Only blit neutral surfaces.
  * Units:
    * Increased the experience requirement for the Rami from 32 to 39
    * Increased the experience requirement for the Saree from 56 to 64

src/sdl_utils.cpp

@@ -1908,11 +1908,14 @@ surface create_compatible_surface(const surface &surf, int width, int height)
 	return s;
 }
 
-void blit_surface(const surface& src,
+void blit_surface(const surface& surf,
 	const SDL_Rect* srcrect, surface& dst, const SDL_Rect* dstrect)
 {
-	assert(src);
+	assert(surf);
 	assert(dst);
+	assert(is_neutral(dst));
+
+	const surface& src = is_neutral(surf) ? surf : make_neutral_surface(surf);
 
 	// Get the areas to blit
 	SDL_Rect dst_rect = create_rect(0, 0, dst->w, dst->h);

src/sdl_utils.hpp

@@ -386,7 +386,7 @@ surface create_compatible_surface(const surface &surf, int width = -1, int heigh
  * The rectangles are const and will not be modified.
  *
  * @pre @p src contains a valid canvas.
- * @pre @p dst contains a valid canvas.
+ * @pre @p dst contains a valid neutral canvas.
  * @pre The caller must make sure the @p src fits on the @p dst.
  *
  * @param src          The surface to blit.