repr trivium
clip() can now use image masks

- with no other arguments, uses either alpha or blackness based on mask
image type
- the `channel` arg can be “white”, “black”, “red”, “green”, “blue”, or
“alpha”. when a channel is specified, high values in that channel map
to high alpha in the clipping block
- the `mask` arg toggles the draw-through behavior so low values map to
high clipped alphas instead
added clipping helper methods to Image

- also rewrote the _draw method to be less out of control w/r/t
transform state
stopped shadowing the transform property in Bezier

- more scaffolding coming down