Store a per-mip-map initialization bit for texmaps

We have been preventing texmaps from getting initialized twice, but that
restriction is per-mip-map, not per-texture. As a result we'd accuse you of
trying to initialize twice if you initialized mip map 0 and then tried to
initialize mip map 1.

But no more!

Signed-off-by: Casey Dahlin <cdahlin@redhat.com>
Don't invert meaning of TEXMAP_STENCIL

Signed-off-by: Casey Dahlin <cdahlin@redhat.com>