mesa: new driver meta-ops module

mesa: new driver meta-ops module

Implement glClear() in terms of quad rendering, implement glBlitFramebuffer()
in terms of glCopyTexImage2D + textured quad, etc.

There have been several places in the drivers where we've implemented
meta rendering similar to this.  This is an effort to do it in a more
portable and more efficient form.

The _mesa_meta_begin/end() functions act like glPush/PopAttrib() but are
lighter-weight.  Plus, _mesa_meta_begin() resets GL state back to default
values (texturing off, identity vertex transform, etc) so the meta drawing
functions don't have to worry about it.

For now only _mesa_mesa_blit_framebuffer() and _mesa_meta_clear() are
implemented.  glDrawPixels() and glCopyPixels() would be the next candidates.