Fixed bug 3159 - SDL_blit_slow with BLENDING does not work

Fritzor

Source Suface is ABGR and Destination Surface is ABGR. We use software blending. In the Switch-Case statement for SDL_COPY_BLEND (Line 126) the alpha-channel is not calculated like in every SDL_blit_auto - function. So if the destination Surface has alpha - channel of zero the resulting surface has zero as well.
Add:  ?dstA = srcA + ((255 - srcA) * dstA) / 255;? to code and everything is okay.

src/video/SDL_blit_slow.c

@@ -129,6 +129,7 @@ SDL_Blit_Slow(SDL_BlitInfo * info)
                 dstR = srcR + ((255 - srcA) * dstR) / 255;
                 dstG = srcG + ((255 - srcA) * dstG) / 255;
                 dstB = srcB + ((255 - srcA) * dstB) / 255;
+                dstA = srcA + ((255 - srcA) * dstA) / 255;
                 break;
             case SDL_COPY_ADD:
                 dstR = srcR + dstR;