mesa: Add new constants related to GL_ARB_viewport_array

mesa: Add new constants related to GL_ARB_viewport_array

These limits will be queryable by GL_MAX_VIEWPORTS,
GL_VIEWPORT_SUBPIXEL_BITS, and GL_VIEWPORT_BOUNDS_RANGE.  Drivers that
actually implement the extension must set values for these constants
that comply with the minimum-maximums from the spec.

Most of these changes were part of other patches.  They were separated out
because it make reordering of later patches easier.  Also, MaxViewports wasn't
set by that patch, and I completely overlooked it in review.  It's now obvious
that it's set. :)

v2 (idr): Split these changes out from the original patches.  Keep
MaxViewportWidth and MaxViewportHeight as GLuint.

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>