Consider using make_surface_current_tiles._apply_pngquant() for other image loop PNGs

[douglatornell]

Original report by Doug Latornell (Bitbucket: douglatornell, GitHub: douglatornell).

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@mdunphy commented in https://bitbucket.org/salishsea/salishseanowcast/pull-requests/6:

The surface current tiles are low-colour figures, ostensibly four colours used but there are a
bunch of gradient colours due to anti-aliasing, etc. The figures are visibly indistinguishable 
after compressing to a 16-colour palette with pngquant, and this reduces size by about a 
factor of 5. Thus the animations should load faster.

This approach would work for the other tracer image loops as well, but should use a larger 
number of colours (perhaps 256)