Support desktop (click and drag) rotation

[JaffaKetchup]

Resolves #1568.

Thanks to @TesteurManiak for the infrastructure implementation and POC and @ibrierley for the rotation algorithm itself. Also thanks to https://stackoverflow.com/questions/48916517/javascript-click-and-drag-to-rotate and https://github.com/openlayers/openlayers/blob/7099f14bd2348491c1550f7b0be929e5da00d83d/src/ol/interaction/DragRotate.js#L65.

Still to do before this can be release ready:

• Ensure this does not break other existing gestures
• Allow for other keyboard triggers
• Add InteractiveFlag.cursorRotate (redundant with 'Allow for other keyboard triggers')
• Consider renaming InteractiveFlag.rotate for clarity (InteractiveFlag.cursorRotate will not be added)
• Allow for customizing rotation degree scale (1 by default) (does not work correctly and can cause rotation jumps)

Also moved InteractionOptions.enableScrollWheel to InteractiveFlags.scrollWheelZoom with deprecation.

[ibrierley]

I only included the round function as that was in the original solution I was looking at. I never had chance to do any detailed digging into whether it causes any issues without it. I did ponder it at the time and couldn't really figure why it was necessary.

[TesteurManiak]

Just got some very minor remarks, otherwise LGTM! 👍

[androidseb]

Thank you very much for adding in this feature! I've tested it just now on the demo page here:
https://demo.fleaflet.dev/

For me it works mostly fine, but I noticed the rotation jumps around a little bit in unexpected ways (maybe I don't have the correct expectations?)...

Here are some reproduction steps of the issue in case this helps:

• Load the demo page here in the latest version of chrome (Version 116.0.5845.140 (Official Build) (64-bit)): https://demo.fleaflet.dev/
• Hold the "Ctrl" keyboard key
• With the left mouse button, drag the map from the top left corner down by just a few pixels
• UX issue:
  • EXPECTED: the map rotates by just a few degrees
  • ACTUAL: the map is rotated instantly by 45 degrees, then by a few additional degrees

It appears wherever the mouse is dragged to relative to the center of the map view becomes the new north of the map. @JaffaKetchup Is this how it's expected to work? I would have expected the map rotation to allow us to "Drag" the map for rotation by a few degrees at once, not set the new north.

[JaffaKetchup]

Hi @androidseb,
Indeed, this is the way it's supposed to function at the moment, but I do agree it might be better if we could just apply an offset to the current rotation, instead of setting the rotation outright. I'll see what I can do, but if I can remember correctly, trying to implement it like this caused issues.