glamor: Always allocate precisely the requested pixmap size

Using a pixmap as a tile or stipple means that we must have the
underlying FBO match the pixmap geometry exactly. We may want to add
some complexity here to migrate pixmaps into exact sized objects as
necessary, but for now, make the server work correctly by skipping
this optimization.

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Eric Anholt <eric@anholt.net>