More snapping angles for straight lines

I've worked on some trimetric games with gradient of 1/3 and 1/4. Pyxel currently only allows snapping to 45 degree and 1/2 gradients.

Furthermore, it would be useful for the 2 pixel gradient lines and other to be able to fill a single pixel of a step at a time, whereas presently it only adds pixels in sets of 2.