fix a bug where fading in/out of opaque 32-bits windows wasn't working

opaque 32-bits windows are now allocated as RGBX_8888 buffers and
SurfaceFlinger always uses GL_MODULATE instead of trying to
optimize to GL_REPLACE when possible (makes no sense on
h/w accelerated GL).

we still have a small hack for devices that don't support
RGBX_8888 in their gralloc implementation where we revert to
RGBA_8888.