mesa: Update gl_scissor_attrib to support ARB_viewport_array

mesa: Update gl_scissor_attrib to support ARB_viewport_array

Update Mesa and drivers to access updated gl_scissor_attrib.
Now have an enable bitfield and array of gl_scissor_rects.
Drivers have been updated to the new scissor enable state
attribute (gl_context.scissor.EnableFlags) but still treat it
as a single boolean which is okay as mesa will only use
bit 0 when communicating with a driver that does not support
ARB_viewport_array.

v2 (idr): Rebase fixes.

v3 (idr): Small code formatting fix suggsted by Ken.

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>