st/mesa: fix texture deletion context mix-up issues (v2)

st/mesa: fix texture deletion context mix-up issues (v2)

When we destroy a context, we need to temporarily make that context
the current one for the thread.

That's because during context tear-down we make many calls to
_mesa_reference_texobj(&texObj, NULL).  Note there's no context
parameter.  If the texture's refcount goes to zero and we need to
delete it, we use the thread's current context.  But if that context
isn't the context we're tearing down, we get into trouble when
deallocating sampler views.  See patch 593e36f956 ("st/mesa:
implement "zombie" sampler views (v2)") for background information.

Also, we need to release any sampler views attached to the fallback
textures.

Fixes a crash on exit with a glretrace of the Nobel Clinician
application.

v2: at end of st_destroy_context(), check if save_ctx == ctx and
unbind the context if so.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Neha Bhende <bhenden@vmware.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>