add line art (edge detect) comparison mode

[ontl]

Here's a pitch for adding line art functionality. It adds another offscreen canvas where we process the existing images to generate colorized outlines, before adding them to the primary canvas.

In future it could be improved with a canvas filter or webGL implementation if performance turns out to be too rough, but it seems to work ok in my testing.

I've included an optional default for blending mode depending on whether Photo or Line mode is selected (it's currently commented out).

Thanks!

[louis-ev]

This is fantastic! And very useful, thanks :)

I think I'll add:

• a toggle up top to make this easier to enable
• a unique static color for each bike (arbitrary, but close to the frame color) to make them easier to identify