PPU: implement OAM DMA transfer

[taylus]

Writing to $FF46 should cause a bulk copy of memory into OAM.

See:

• http://bgb.bircd.org/pandocs.htm#lcdoamdmatransfers
• https://github.com/Two9A/jsGB/blob/9b93fd1644fa83208d1a22df4e27a0e5903d0698/js/gpu.js#L382
• https://github.com/AntonioND/giibiiadvance/blob/master/docs/TCAGBD.pdf
• http://www.codeslinger.co.uk/pages/projects/gameboy/dma.html

[taylus]

Dr. Mario is a good example of using this since it doesn't require a memory mapper to run.

Its vblank handler jumps to $01a6. At $01c0 is a call $ffb6 which is in HRAM so it must be the game's DMA transfer routine:

See also: https://github.com/taylus/gameboy-dev/tree/master/rgbds/03-hello-sprites

This is why when I run this ROM currently, no heart sprite appears on the title screen...! (sprite OAM is empty... all zeroes, because DMA isn't filling it in).

[taylus]

#29 closes this