enable working with multiple levels of drawings

[monking]

Keep it as simple as possible. I'll have a little discussion with myself below to work out the first approach, and then put the features here

after #24

• click on an exposure in the timeline to draw there
• add a button to start a new level in the same exposure

until lines are variable, the order of the levels doesn't matter. changing levels can happen later.

[monking]

How to add and remove levels?
Just keyboard shortcuts to start?

How to represent how many levels you have, which one you're on?
Show the strokes on all other levels as some light gray?

Does this beg to add fills?

Should the user be able to change the order of levels?

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I'm getting ahead of myself. What are the main reasons to introduce levels?

1. separate levels to sketch, fill, and ink
2. give different levels a different number of exposures, to enable still backgrounds or animating only a part of a character
3. animate whole levels to simulate camera moves or to make an animatic

The first one suggests 3 fixed levels could be offered, with a little radio select in the status

To tackle different frame exposures, I need to work out the file format. I was originally going to put levels under frames. I should instead put them in a different structure, and have each stroke reference which level it's on.

A simple linear tween of a level may rarely be useful. It might make more sense to implement this in WebGL and give real depth to the drawings, then only tween the camera. That's barely on my horizon right now.

I think I'll first implement frame copying, which will accomplish some of the same goals, and frames can be used for sketching, inking done on a following frame, and then the sketch frame discarded