mesa: Add new scissor entry points for GL_ARB_viewport_array

mesa: Add new scissor entry points for GL_ARB_viewport_array

v2 (idr): Use set_scissor_no_notify (and manually notify the driver)
instead of calling _mesa_set_scissori.  Refactory bodies of
_mesa_ScissorIndexed and _mesa_ScissorIndexedv into a shared function.
Perform parameter validation in the same order in all three functions.
Pull MaxViewports comparison fix (in _mesa_ScissorArrayv) from the next
patch to this patch.

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>