video/s3virge: refactor

[angelosa]

(TBD)

[angelosa]

Opening as PR for checking where I fumbled during the conversion (timer drift?).

[angelosa]

I don't yet have a well defined goal here, shortening the FIFO queue is challenging, namely because the docs don't exactly define what's a "slot" vs. the actual BitBLT command length. This is important for win2k (which hangs as soon as user starts accessing stuff) and for a couple (minor) corruption bugs still happening in win98 land.
May consider just doing a bare std::function conversion in selected BitBLT places + other misc cleanups and call it for this branch, postponing localized fixes at later date unless something really obvious pops up.

[angelosa]

Closing this, applied with 3b5e60b , no use having this in WIP and as a feature branch.