[DOS/EGA] Performance Issues

[sulix]

The DOS/EGA renderer is way slower than the original Keen one, which causes problems both on real hardware (it's slow-but-playable on a 486 with a fast VLB card, but nigh unusuable on a 40Mhz 386 with an ISA VGA card), and potentially under DOSBox (where 3000 cycles is way too slow).

There are a few obvious optimisations we can attempt:

1. Actually move the tile buffer into video memory. This will speed up erase blocks and animated tiles, as we can copy 4 bytes at a time.
2. As a result, the tile buffer can scroll properly, rather than being copied around. This should drastically increase scrolling speed, which is the real bottleneck.
3. Write optimised blitters and call them directly. There's a lot of conditional code an indirection to pick both the VL_DOS backend, then check both source and destination surface type.
4. (As a bonus, add optimised 16×16 px aligned tile blitters, which would be much faster for the common case, and a 'scale' blitter or similar for the SW-scroller, which is absurdly slow. Maybe we'd need the scaler compiler, too, but baby steps.)
5. If we split up the functions which write to EGA memory, we can more easily support DJGPP farmem instead of nearmem (slower, but WinNT compatible), and maybe non-SVGA-compat mode wrapping. (The latter may be needed for a scrolling tile buffer anyway).
6. If we're still too slow, we can maybe investigate doing tilebuffer→tilebuffer copies when scrolling on a new tile which is already onscreen. The cache metadata would probably take enough memory and be slow enough to only make this worthwhile on very slow ISA busses, though.