Fix compositing for premultiplied alpha textures
Fix compositing for premultiplied alpha textures

When the texture has premultiplied alpha, glBlendFunc
needs to be GL_ONE, GL_ONE_MINUS_SRC_ALPHA to avoid artifacts.

nemomobile-graveyard/mcompositor,nemomobile-graveyard/mcompositor,nemomobile-graveyard/mcompositor,nemomobile-graveyard/mcompositor,nemomobile-graveyard/mcompositor,nemomobile-graveyard/mcompositor