Add Surface::get_pixel and remove special case from blit code

[Daft-Freak]

After repeating my "blit that is actually a fill" optimisation for the third time, I worked out a way to avoid it entirely and improve TileMap perf a bit. Also adds a useful get_pixel method to Surface (returns the pixel converted to a Pen).

I haven't benchmarked the change to stretch_blit, but I'd expect a similar effect there.

(Making the blit code smaller is also a good thing for the firmware)

[Gadgetoid]

Wow, I can't even wrap my brain around what the edge case was doing and why, but it was... janky 😬

Thanks!