Move createSkinTexture out of demo
Change demo clear color to a pleasant sky blue background (vs black)

Mainly so the meshes are still visible if the textures fail to load (for
example, if you run the demo locally, not through a localhost web
server, and the local file loading is denied), as solid black, and it
provides a better contrast against most skins instead of solid white.
Refactor skin conversion into copyPixels()
Support 64x64 skin left leg/arm, copy for 64x32. Closes GH-8
Restore support for high-res 64x32 skins, when converting to 64x64 format
Attempt to copy left arm in reverse, but it looks like each side needs to be individually flipped (and this code has no effect)
Add copyPixelsFlipX and simplify copyPixels
Mirror arm when converting skin 64x32 to 64x64. Closes GH-8

Previously the left arm was mirrored from the right arm by setting the
UV coordinates backwards, but now it is mirrored in the image file.