Fixed alpha composition when destination alpha is transparent.

Jianyu Guan

I found I make a big mistake that when dstA==0, I just simply let *dstp=*srcp and forgot to make dstRGB = srcRGB*srcA.

The if consition "(*dstp & amask) == 0" in BlitRGBtoRGBPixelAlphaMMX and BlitRGBtoRGBPixelAlphaMMX3dNow should be removed.
[Bug 2042] OpenGL ES renderer tries to load OES functions unconditionally

Also, fail more gracefully when creating texture to avoid double free errors.
Patched to compile.