intel: Enable S3TC extensions always

intel: Enable S3TC extensions always

Always enable the use of pre-compressed texture data.  The ability to
perform on-line compression still requires the presence of libtxc_dxtn
or an explicit driconf over-ride.  Applications that just want to submit
precompessed data when an on-line compressor is not available can look
for the GL_EXT_texture_compression_dxt1 and
GL_ANGLE_texture_compression_dxt[35] extensions.

v2: Only enable the extensions that do not require on-line compression
by default.  The previous statement "This should not impact many (if
any) real applications." proved to be false for at least Sauerbraten.
This application mostly submits pre-compressed data, but it also can
submit uncompressed data that it asks the driver to compress.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com> [v1]
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org> [v1]
Acked-by: Eric Anholt <eric@anholt.net> [v1]
Acked-by: Lee Salzman <lsalzman@gmail.com>