Implement CRTC "max raster raster" register

[giomba]

The max raster raster CRTC register is used to configure how tall a character is (eg. Sanco ROM configures it as 0xD).
But the character ROM holds character up to 16 (0x10) lines tall.

Changing the max raster raster register affects:

• the video module, eg. what does vertical stretch do? check
• also, check the documentation in the ceda-home repository about stretch, because it may be wrong
• is useful for the CGE mod
• is not actually implemented correctly in the emulator because I did not understand what it does (and still I am not 100% sure)

Since the video module is starting to get a bit complex, and in order to avoid future regressions, it would be nice to have a dedicated command to do a video memory dump, and then compare it to a known one (pixel-by-pixel) in a test.