intel: Try using glCopyTexSubImage2D in _mesa_meta_BlitFramebuffer

intel: Try using glCopyTexSubImage2D in _mesa_meta_BlitFramebuffer

In the case where glBlitFramebuffer is being used to copy to a texture
without scaling it is faster if we can use the hardware to do a blit
rather than having to do a texture render. In most of the drivers
glCopyTexSubImage2D will use a blit so this patch makes it check for
when glBlitFramebuffer is doing a simple copy and then divert to
glCopyTexSubImage2D.

This was originally proposed as an extension to the common meta-ops.
However, it was rejected as using the BLT is only advantageous for Intel
hardware.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=33934
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>