MEGA65: implement line-drawing DMA mode

[lgblgblgb]

As part of issue #198, the main and most important missing piece of DMA emulation is "line drawing mode" (LDM for short from now on).

Till now, the main bottleneck to implement was my mental limitation to understand how this works at all :sweat_smile: OK, ok, time was a factor as well not to try to dig into the VHDL of mega65-core ... :sweat_smile:

Fortunatelly @bjotos (btoschi on MEGA65 Discord) wrote a test implementation in C, which was shared with me, huge thanks!

As I thought before, the major problem is understanding how the address incrementing works where the mentioned work before helps.

Original MEGA65 topic on implementing LDM: https://github.com/MEGA65/mega65-core/issues/290