Finally (reasonably) texture mapping

• The existing `surfaceSTToTexelUV()` implementation was completely wrong.  Now finds the nearest point between points 1 & 4 and between 2 & 3, then the nearest point between those nearest points (which should be close enough), and then uses the ratios along the segments horizontally and vertically as the U and V coords.
	» Based on a loose and understanding of Wikipedia's article on Bilinear interpolation, which is probably not the best algorithm for this job.  (Discovered a little too late that barycentric coordinates are what I’m after.)
	» Now it’s just somewhat wrong— it has a strange warp in it that follows the shape of it and looks cool, but also isn’t really how standard texture mapping works.  Good enough to given an in-game shot with though.
	• New `ratioAlongSegment()` function, derived from a few similar sources for closest-point-on-a-line-segment algorithms.
• Fix: Subtracting half a pixel from each dimen in the `nearestTexelXY` calculation in order to effectively use the middles of pixels, not top-left corners.
Bonus: README w/ screenshot!

• `README.md` w/ a minimal write-up.
• `README.screenshot.ipad-vertical.png` of the app in action.