[Patch] Fix 90° offset of `icon-rotation: way;` in mapcss

[Woazboat]

The (undocumented) *-rotation: way;in mapcss has an offset of 90° compared to *-rotation: <fixed angle>

The 90 degree offset occurs because https://josm.openstreetmap.de/browser/josm/trunk/src/org/openstreetmap/josm/tools/RotationAngle.java#L37 uses Geometry.getSegmentAngle, which calculates the angle using Math.atan2 and therefore the rotation starting from the x-axis (east) in counter clockwise direction. The icon rotation is relative to the icon and therefore 'starts' at the y-axis (north) going in the clockwise direction.
The returned angle is already correctly inverted to fix the rotation angle, but the 90° offset is not corrected. This can be fixed by adding an offset of - Math.PI/2 to the angle (+Math.PI/2 if done after inversion).

​https://github.com/JOSM/josm/pull/109
​https://github.com/JOSM/josm/pull/109.patch

See also #22695

[taylor.smock]

Any chance you can write a test for it? I can if you don't have the time.

[Woazboat]

I added some tests to the PR

[taylor.smock]

In 18661/josm:

Fix #22703, see #22695, #22704: 90° offset of icon-rotation: way; in mapcss (patch by Woazboat, modified)

This is due to Geometry.getSegmentAngle getting the rotation from the positive
x-axis instead of the positive y-axis, but this is easily corrected by
subtracting 90 degrees (pi/2) from the calculation.

The previous behavior differed from the static rotation angle behavior, which
uses the positive y-axis.