mesa: rework texture completeness testing

mesa: rework texture completeness testing

Instead of gl_texture_object::_Complete there are now two fields:
_BaseComplete and _MipmapComplete.  The former indicates whether the base
texture level is valid.  The later indicates whether the whole mipmap is
valid.

With sampler objects, a single texture can appear to be both complete and
incomplete at the same time.  See the GL_ARB_sampler_objects spec for more
details.  To implement this we now check if the texture is complete with
respect to a sampler state.

Another benefit of this is we no longer need to invalidate a texture's
completeness state when we change the minification/magnification filters
with glTexParameter().

Reviewed-by: José Fonseca <jfonseca@vmware.com>
Reviewed-by: Eric Anholt <eric@anholt.net>