Fix image_to_term256 for images with alpha
Lock Canvas.trans to None
Add trans support to PilImage
Add trans support to PngImage
Add docs for trans
Merge `trans` support for images

See https://github.com/bjorn/tiled/wiki/TMX-Map-Format#image
assert_png_repr_equal (tests): Add ε, and add more stats to output
Canvas: Handle drawing images with opacity < 1
Restore Python 3 compatibility