Dark outline around dragged sprites

[adroitwhiz]

Expected Behavior

Sprites should have the same appearance when being dragged as when they're not being dragged

Actual Behavior

If you drag a sprite in the editor, a dark outline appears around it:

Note that this was not introduced by #515--as far as I can tell, it's always been a problem.

The problem is that extractDrawable returns data from gl.readPixels, which is premultiplied (and has been even before #515, which only moved the premultiplication out of the shader and into the texture upload). All DOM APIs which consume image data, on the other hand, expect non-premultiplied ("straight") alpha.

My first premultiplication PR (#460) added a new draw mode which "un-premultiplies" the alpha of drawn sprites, which I left out of the second in order to reduce the scope of the changes. I'll probably end up making a new PR with said "un-premultiplying" draw mode.

Steps to Reproduce

1. Create a lightly-colored sprite
2. Drag it