Summary

1. Added proper saturate MIXes.
2. FIFO now gets cleared upon starting an accel command as well as reading it in the appropriate ports (0x9ae8/0xdae8 for Standard FIFO and 0x9aee for Extended FIFO). This fixes slowness in the built-in ATI NT 3.1 drivers.
3. Made a custom loader to the add-on bios so that it can operate on 0xxx800 bios addresses without crashes.
4. When the mach8 add-on bios is enabled alongside an ATI 18800/28800 card, make sure the latter chip's clock are not conflicting with the mach8 clocks. Fixes segfault when making this combination.
5. Default the SRC Y Direction port to 1 for proper operation on chip initialization and reset. Fixes some drivers on Windows 3.x.
6. IBM 8514/A Line draw using vector line (bit 3), when nibble mode is enabled with the pixel control register set to select the foreground register, make sure it doesn't NULL the mix data selection. Fixes glitches under NT 3.1's ATI drivers.

Checklist

• Closes ATI Mach32 slow graphics and some glitches #6927
• I have tested my changes locally and validated that the functionality works as intended
• I have discussed this with core contributors already
• This pull request requires changes to the ROM set
  • I have opened a roms pull request - Update the mach8 add-on rom. roms#476
• This pull request requires changes to the asset set
  • I have opened an assets pull request - https://github.com/86Box/assets/pull/changeme/

References

Provide links to datasheets or other documentation that helped you implement this pull request.