There are a couple of issues with the selection of Altivec alpha-blitting
routines in CalculateAlphaBlit() in src/video/SDL_Blit_A.c.

1) There's no check for the presence of Altivec when checking if the
Blit32to565PixelAlphaAltivec() routine can be selected.

2) Altivec cannot be used in video memory, and there's no check if the
destination surface is a hardware surface. (Alpha-blitting to a hardware
surface with GPU support is a bad idea, but somebody's bound to do it anyway.)

Patch to fix these attached.

--HG--
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/trunk%401243