DMAgic line mode fixes and improvements

[MassiveBattlebotsFan]

Changes:

• Adds line scaling support via the source and destination skip rate extended DMA list options.
  • The bytes and 256ths registers are treated as the MSB and LSB of a 16-bit fractional value.
  • Not setting the skip rates or setting the skip rates to $0000 for the corresponding line mode will not scale the line.
  • Should complete Feature Request - DMA Line fetch  #401.
• Fixed a bug in DMAgic's line mode startup delay that caused count to be decremented too many times (resolves DMA Line Drawing - Calculated length is incorrect. #873).
  • This also fixes weird slope behavior in the first line section.

[lydon42]

Please also test MEGAmaze against this. It's on INTRO Disk 1.

[MassiveBattlebotsFan]

Bringing out of draft because this now functions as expected. Line mode scaling must now be enabled with extended option $8F/$9F bit 4 (previously unused) for compatibility.