mesa: fix a render to texture FBO validation bug

mesa: fix a render to texture FBO validation bug

When glTexImage() is called we need to re-validate any FBOs that point to
the texture (i.e. render-to-texture) since changing the texture's size/format
will effect FBO completeness.

We don't keep a list of all FBOs rendering into each texture (which would be
a bit messy) so we check all FBOs in existance.  To optimize this, the
gl_texture_object->_RenderToTexture flag is used to avoid checking textures
that have never been used as renderbuffers.  So, we only walk over all FBOs
(there's usually only a few) when glTexImage() modifies a RTT texture.

Fixes a bug seen in shadowtex.c when toggling packed depth/stencil mode.