Port ToaruPaint to Kuroko

ToaruPaint (painting.py) was a drawing app available in ToaruOS Legacy. The version in the last release featured the ability to zoom and pan a canvas, adjust the brush size by scrolling the mouse wheel, and select colors with a color picker. Another Python graphical application from ToaruOS Legacy, mines.py, was successfully ported to Kuroko and the newer in-house graphics libraries; it would be nice to give ToaruPaint the same treatment.

The color picker in ToaruPaint was rewritten in C already; it might be worthwhile to restructure it as a library that can then be exposed to Kuroko so we can reuse it.

The majority of the drawing code in ToaruPaint used Cairo to paint simple stroked line segments to a texture and then draw that texture, scaled and translated, to the window. All of this is easily replicated in our own graphics libraries; even drawing the repeated pattern of the checkerboard background is now possible through libtoaru_text's textured shape rasterization.

One thing we are missing is the ability to export images as PNGs, but following the example set in the compositor we can write output as a Targa image instead.