mesa: Set all scissor rects

mesa: Set all scissor rects

In _mesa_Scissor, make sure that ctx->Driver.Scissor is only called once
instead of once per scissor rectangle.

v2: Use MAX_VIEWPORTS instead of ctx->Const.MaxViewports because the
driver may not set ctx->Const.MaxViewports yet.

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