glamor: Don't try to set up core fonts textures when we won't use them.

This happens to avoid GL errors on hardware without
EXT_texture_integer (which implies < GLSL 130, and thus glamor_text.c
programs not compiling anyway).

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Keith Packard <keithp@keithp.com>