Improve global predictions performance

[ahocevar]

This pull request improves the rendering performance of the global predictions layer:

• Redefine view zoom levels so zoom 0 is the Web Mercator world on a 512x512 pixel tile. This is what Mapbox/MapLibre folks are used to.
• Hint the fields layer to have no overlaps (speeds up rendering by making use of larger batches)
• Improve the style by reusing style objects and pre-calculating colors
• Debounce the confidence slider (like in the MapLibre proof-of-concept)
• Do not fill tiny polygons where the stroke would not be visible anyway, because it's less than 1 pixel in size
• Use cheaper butt, miter and miter limit settings for strokes

These changes speed up rendering by approximately a factor of 10 on my machine.

If people still feel there is too much lag when panning or changing the confidence, I can make another change that decouples rendering from the main thread, so the UI is not blocked at all any more.

[m-mohr]

This changes the couple of seconds unresponsiveness (depending on area) to pretty much no unresponsiveness for me (depending on area).

"Calebs example in the US" is responsive throughout, "my north nile example" has a bit of stutter while panning but not much.

When we can move some of the COG + PMTiles rendering into workers, this could solve it altogether.