Altivec-optimized blitters!

Vast majority of this work is compliments of Bob Ippolito.

http://www.devolution.com/pipermail/sdl/2005-February/067466.html and many
 other posts.

--HG--
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/trunk%401048

src/video/SDL_blit.h

@@ -374,6 +374,20 @@ do {						\
 	dB = (((sB-dB)*(A))>>8)+dB;		\
 } while(0)
 
+/* Blend the RGB values of two pixels based on a source alpha value */
+#define ACCURATE_ALPHA_BLEND(sR, sG, sB, sA, dR, dG, dB)	\
+do {						\
+    unsigned tR, tG, tB, tA; \
+    tA = 255 - sA; \
+    tR = 1 + (sR * sA) + (dR * tA); \
+    dR = (tR + (tR >> 8)) >> 8; \
+    tG = 1 + (sG * sA) + (dG * tA); \
+    dG = (tG + (tG >> 8)) >> 8; \
+    tB = 1 + (sB * sA) + (dB * tA); \
+    dB = (tB + (tB >> 8)) >> 8; \
+} while(0)
+
+
 /* This is a very useful loop for optimizing blitters */
 #if defined(_MSC_VER) && (_MSC_VER == 1300)
 /* There's a bug in the Visual C++ 7 optimizer when compiling this code */