Fixed SDL_SetAlpha compatibility function to match the documentation

--HG--
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/trunk%403299

src/SDL_compat.c

@@ -686,16 +686,19 @@ SDL_GetVideoSurface(void)
 int
 SDL_SetAlpha(SDL_Surface * surface, Uint32 flag, Uint8 value)
 {
-    if (flag & SDL_RLEACCEL) {
-        SDL_SetSurfaceRLE(surface, 1);
+    if (flag & SDL_SRCALPHA) {
+        /* According to the docs, value is ignored for alpha surfaces */
+        if (surface->format->Amask) {
+            value = 0xFF;
         }
-    if (flag) {
         SDL_SetSurfaceAlphaMod(surface, value);
         SDL_SetSurfaceBlendMode(surface, SDL_TEXTUREBLENDMODE_BLEND);
     } else {
         SDL_SetSurfaceAlphaMod(surface, 0xFF);
         SDL_SetSurfaceBlendMode(surface, SDL_TEXTUREBLENDMODE_NONE);
     }
+    SDL_SetSurfaceRLE(surface, (flag & SDL_RLEACCEL));
+
     return 0;
 }