Unlike the hardware backends, the software renderer can use multiple scissor rectangles (though this will result in extra rasterization).

This is needed since we need a separate offset for zfreeze to work correctly.  It also makes the code a bit less jank.

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.