Image processing

[stuartpb]

It'd be kind of useful to just have a "change the hue of this image" or "invert this image's colors" thing you could do (alongside stuff like JSON patching). Feasible, too, since it's doable just via canvas operations.

[stuartpb]

So, the four on the road map:

• filter: Ideally, this could just apply a CSS filter to the whole image - but right now, CSS filters on images in Chrome are in some kind of limbo, so this is going to have to wait a bit (will probably get its own issue for tracking progress).
• setColor: Sets the entire image to the given color (ie. hue and saturation).
• fillColor: Fills all alpha in the image with the given color, for solid-color images.
• setHue: Sets the entire image to the given hue, in degrees.

[stuartpb]

Everything but filter is done - this issue is now for tracking progress on that.

[stuartpb]

filter depends on CanvasRenderingContext2D.filter, which appears to have been implemented in Chrome 52 - the version that got promoted to the stable branch July 20, so: full steam ahead on that (and deprecating the old rotate-hue-180 hack, if I ever implemented it).