Switch to inline colourable markers

[hlfan]

Description

This is an attempt at parametrifying marker colors with currentColor since it has come up in #5764.
Together with optimizing the path geometry, this required rewriting the marker layout to be compatible with partially transparent gradients.
While an even simpler path has been proposed and demonstrated in https://antonkhorev.github.io/msvgen, I was searching for a solution without a sudden change in curvature, which I found by constraining a Bezier curve to a specified slope, 1.92 in this case.¹ (See CodePen)

Where I had the source colors, I tried to fit the colors to the sources, e.g. for the note markers see this before/after:

For now, this is only a barebones switch to the new markers, but moving in the direction of #872 (comment), #2787 or #507

Closes #5764.

How has this been tested?

Server side and client/browser side separately, no integration testing though.

Considerations

For the raster icons where I didn't have the colors, I took them from the note markers, so

• either I need better sources than I could find, or
• the dot markers will differ in color a bit more:
  

Footnotes

1. Leaflet's marker.svg actually has two slopes for the tip, 1.9205 on the left and 1.9296 on the right ↩

[hlfan]

@tomhughes do you think it's better to match the current marker colors more closely or to reduce variations in color (only have one green, one red...)?

[tomhughes]

I couldn't see that much of a difference to be honest - there was a bit of one with the routing markers but nothing terrible.

I know this overlaps with work @AntonKhorev has been doing though which is why I've asked him to review it.

[tomhughes]

The only other thing that struck me was the various hex colour constants scattered around but I'm not sure there's any good way to centralise them.