mesa: Fix GL error generation in glBlitFramebuffer()

mesa: Fix GL error generation in glBlitFramebuffer()

V2:
If mask has GL_STENCIL_BUFFER_BIT set, the depth formats for
readRenderBuffer and drawRenderBuffer must match unless one of the two
buffers doesn't have depth, in which case it's not blitted, so the
format check should be ignored.  Same comment goes for stencil formats
in depth renderbuffers if mask has GL_DEPTH_BUFFER_BIT set.

v3 (Kayden): Refactor code to be a bit more readable.

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>