Use blit_surface() instead of SDL_BlitSurface() in the ~BG() implementation

For some reason, ~BG() used SDL_BlitSurface() directly instead of using
the alternate blit_surface() implementation, unlike every other image
path function type in existence that needs to do blitting.

This works the first time when scaling or cropping an image (say,
"misc/blank-hex.png~BG()~CROP(0,0,34,34)"), but the second time it's
used with different scaling/cropping parameters, it yields the result it
would without ~BG() in the pipeline. This is somehow remedied by this
change, and I don't see any potential issues arising from it anyway
(save for a negligible performance impact on ~BG(), maybe).

changelog

@@ -18,6 +18,8 @@ Version 1.13.0-dev:
    * Increased the experience requirement for the Saree from 56 to 64
  * Miscellaneous and bug fixes:
    * Fix Fisher-Yates implemenation of Lua helper.shuffle (bug #21706)
+   * Fixed issues with the ~BG() image path function not always working with
+     cached images.
    * Idle controlled sides now serialize as human controlled.
 
 Version 1.11.11:

src/image_modifications.cpp

@@ -460,7 +460,7 @@ surface background_modification::operator()(const surface &src) const
 					    color_.b, color_.unused));
 #endif
 	SDL_SetAlpha(src, SDL_SRCALPHA, SDL_ALPHA_OPAQUE);
-	SDL_BlitSurface(src, NULL, ret, NULL);
+	blit_surface(src, NULL, ret, NULL);
 	return ret;
 }