meta: Drop blit src size fallback.

meta: Drop blit src size fallback.

I think we can assert that renderbuffer size is <= maximum 2D texture
size.  Our source coordinates should have already been clipped to the src
renderbuffer size, but haven't actually (so we could potentially have
trouble if there's scaling, and we're in the CopyTexImage path that tries
to use src size).  However, this texture size dependency was blocking the
next refactors, so I'm not sure if we want to go ahead with this series
before we get the clipping sorted out or not.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>