Thank you automated tests (and bobbens!), fixed alpha blending for accuracy

--HG--
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/trunk%404176

src/video/SDL_blit.h

@@ -444,9 +444,9 @@ do {									   \
 /* Blend the RGB values of two Pixels based on a source alpha value */
 #define ALPHA_BLEND(sR, sG, sB, A, dR, dG, dB)	\
 do {						\
-	dR = (((sR-dR)*(A)+255)>>8)+dR;		\
-	dG = (((sG-dG)*(A)+255)>>8)+dG;		\
-	dB = (((sB-dB)*(A)+255)>>8)+dB;		\
+	dR = ((((int)(sR-dR)*(int)A)/255)+dR);	\
+	dG = ((((int)(sG-dG)*(int)A)/255)+dG);	\
+	dB = ((((int)(sB-dB)*(int)A)/255)+dB);	\
 } while(0)