gl/msaa: blit both stencil and color buffer to single sample FBO

msaa compositor for GLESv2 with ANGLE multisample extensions and
GLESv3 uses single-sampled fbo for aliased drawing.  We need to
blit both stencil and color buffer if there is a valid stencil
cache.  Otherwise, we only need to blit color buffer
gl: reduce chances of re-generating mask surface for glyph