perf: implement more DrawTarget methods to improve performance

[zabackary]

There's a couple of changes going on here:

1. I updated vexDisplayCopyRect invocations to properly add 0x20 for the header bar to y2 as well as y1.

2. I modified <DisplayDriver as DrawTarget>::draw_iter to fill individual pixels. I think that was the intention of embedded-graphics and it vastly improves performance.

3. I implemented fill_contiguous which is intended to be used for copying images and such to use vexDisplayCopyRect. I re-used the slice in DisplayDriver to save allocations all the time, but it might be better to just dynamically allocate instead of allocating a buffer of the max size up front.

4. I implemented fill_solid to fill rectanges the usual way.

5. I added methods to update the rendering mode of the display.

Essentially, the original implementation just copied the entire buffer over at once, which wasn't very good for performance since embedded-graphics calls draw_iter several times per frame typically, making for slow frame rates on complex scenes.

[tropicaaal]

LGTM