ARB_fp: accept duplicate precision options

ARB_fp: accept duplicate precision options

Relaxes the validation of

   OPTION ARB_precision_hint_{nicest,fastest};

to allow duplicate options. The spec says that both /nicest/ and
/fastest/ cannot be specified together, but could be interpreted
either way for respecification of the same option.

Other drivers (NVIDIA etc) accept this, and at least one Unity3D game
expects it to succeed (Kerbal Space Program).

V2: Add spec quote.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>