Slight modification of viewport sky: Avoid clearing the colour buffer
since we fill it later anyway. Usually OpenGL does color + depth buffer
concurrently so this probably won't have any noticable effect. Still
better be pedantic about it in case we do earn some performance out of
it. Added alpha component in sky color to make sure it is set to zero in
the framebuffer too.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53609 954f8c5b-7b00-dc11-b283-0030488c597c