A zero source rect is valid for blitting

A zero sized source rect draws pixels sourced from a 0x0 region on the source surface.

Fixes https://github.com/libsdl-org/SDL/issues/8580

src/video/SDL_blit.c

@@ -59,7 +59,7 @@ static bool SDLCALL SDL_SoftBlit(SDL_Surface *src, const SDL_Rect *srcrect,
     }
 
     // Set up source and destination buffer pointers, and BLIT!
-    if (okay && !SDL_RectEmpty(srcrect)) {
+    if (okay) {
         SDL_BlitFunc RunBlit;
         SDL_BlitInfo *info = &src->internal->map.info;
 

src/video/SDL_surface.c

@@ -1226,7 +1226,7 @@ bool SDL_BlitSurfaceScaled(SDL_Surface *src, const SDL_Rect *srcrect, SDL_Surfac
     SDL_GetRectIntersection(clip_rect, &final_dst, &final_dst);
 
     if (final_dst.w == 0 || final_dst.h == 0 ||
-        final_src.w <= 0 || final_src.h <= 0) {
+        final_src.w < 0 || final_src.h < 0) {
         // No-op.
         return true;
     }