Scanline DMA support with optional callback

[matt (gingerbeardman)]

Here's a reference for how it currently works (at the time of Dave posting)

Updating the entire LCD in C and performance

So, I traced on the Saleae and saw that there's a bit of down time each row that makes the difference between 50+ and 48 fps. Makes perfect sense in retrospect: we're only DMAing one row at a time, so there's some turnaround for the interrupt handler to set up the next transfer. I switched to a circular buffer so that it's loading the next row while it's sending the current one and I'm getting just over 54 fps. I feel like I should point out: This doesn't mean that if your game is draggi…