mesa: Allow glGet of values that are 2 doubles

mesa: Allow glGet of values that are 2 doubles

This will be used when the viewport near and far plane are stored as
doubles instead of as floats.

v4 (idr): Split out from a single megapatch.  Suggested by Ken.  Also
drop value_double_4.  It's never used anywhere in the patch series.

Signed-off-by: Courtney Goeltzenleuchter <courtney@LunarG.com>
Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>