Selection Table: reorder colors.

Full blue appears now in place of the light blue.
Red has been pushed further down--generally interferes with presynaptic sites.
Added documentation to tilelayer.js, detailing when and why tile URLS are updated.
tilelayer.js: do not fetch too many image tiles when at scale -1, -2.
tilelayer.js: fix excessive image loading upon resizing the canvas.

Before, too many tiles loaded. One needed to zoom out and then zoom in
to trigger the code that corrected the URLs to a black gif when falling
completely off the field of view.