Added flattenFrameAt to LayerUtils.
Added renderFrameAt to PiskelController (using flattenFrameAt)
Use renderFrameAt in PiskelRenderer (which is used for PNG spritesheet)
chore: renamed createLayerFromSpritesheet to createFramesFromSpritesheet
(in LayerUtils)
Uniform lines step is now capped so that the step remains lower
than min(dx, dy). If the step would be greater than this value,
use Infinity and draw a straight line