[Mesa-dev] Patches to make Gallium drivers respect the force_s3tc_enable environment variable

[Bryan Cain]

The purpose of the following two patches is to make st/mesa expose the S3TC
extensions when the force_s3tc_enable environment variable is used.  This is
to match the behavior of the DRI drivers, in which force_s3tc_enable is an
option in driconf.  Although st/mesa can't use the driconf functions, it can
at least respect the environment variable for this option.

These patches have been tested with fs2_open 3.6.12 and the Freespace 2
MediaVPs.  Setting force_s3tc_enable=true makes the game run flawlessly
without libtxc_dxtn, whereas without it the game writes several error messages
to its log file and then crashes.