mesa: new MESA_EXTENSION_MAX_YEAR env var

mesa: new MESA_EXTENSION_MAX_YEAR env var

If set to year X, only report extensions up to that year.  This is a
work-around for games that try to copy the extensions string to a fixed
size buffer and overflow.  If a game was released in year X, setting
MESA_EXTENSION_MAX_YEAR to that year will likely fix the problem.