Add build option for 9938 VDP

[MirrorPusher]

Implement stripped down 9938 VDP mode as replacement for the F18a.

With that in place it should be possible to use Stevie with a 9938 or 9958 VDP. Also this should allow a version for MAME.
Can be useful for hardcore debugging with MAME, when emulators like js99er and classic99 can't reproduce the bug.

There are no sprites nor TAT tables when using 9938VDP, so textmode only meaning:

• Reduced colors
• No sprites
• No 30 rows mode
• No F18a GPU