Fix wrong alpha channel for OpenGL render results with transparent textures or
materials, the typical glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA); blend
function does not give correct destination alpha.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54638 954f8c5b-7b00-dc11-b283-0030488c597c