Resolve Spring camera flip issues by adding pi when positive and subtracting pi when negative

[TheDujin]

Work done

Investigate engine cardinal lock code and, based on these investigations, devise a better fix for the camera flip widget.

Addresses Issue(s)

#3767

This issue was not fully resolved. While the current widget works when camera ry is set to exactly 0, often, after manually rotating the Spring camera, using Alt+O to flip the camera does not flip the camera by exactly 180 degrees as expected.

Setup

Start a skirmish and toggle to Spring camera (Ctrl+F6).

Test steps

1. Using Alt+middle mouse, set the camera to any rotation. A cardinally locked direction can be tested, and non-cardinally locked directions can also be tested.
   1. If desired, Echo Camera State under Settings->Developer UI can be used to view the value of the camera ry both before and after the camera flip.
2. Press Alt+O to flip camera.
   1. Note that the camera flips perfectly by 180 degrees regardless of whether the original rotation was cardinally locked or not.
3. Repeat steps 1 and 2 with different initial rotations until satisfied.
   1. Verify that when ry is set to very close to 0 (i.e. between -0.1 and 0.1 radians), the camera rotation is still 180 degrees. A camera ry of 0 can be easily obtained by switching to Overhead camera (Ctrl+F5) then switching back to Spring camera (Ctrl+F6).

[TheDujin]

Converting to draft because I realized I haven't handled the "close to 0" edge case.

[TheDujin]

I adjusted the comment. It remains wordy, but delves less into tangentially-relevant technical details of the Engine formula and focuses more on why the code behaves how it does. Is this an improvement or is it worse? 😅

[efrec]

Much improved.

The cardinal lock feature could be recreated game-side easily, which could help in ironing out its quirks. Then, it would be even easier to selectively disable the feature from game code, as well.

I think this is an improvement, regardless, so I'd approve the change. I think this solves the minor issue without getting into the weeds which is welcome.