[EXPERIMENTAL] Remove ThreeDScene and rewrite Camera internals

[chopan050]

• Deleted ThreeDScene and moved many of its methods into Camera. For example, ThreeDScene.begin_ambient_camera_rotation() becomes Camera.begin_ambient_rotation() and ThreeDScene.begin_3dillusion_camera_rotation() becomes Camera.begin_precession().
• All the references to ThreeDScene were changed to Scene.
• Camera was rewritten to work directly with ._theta, ._phi and ._gamma angles rather than using an .orientation quaternion which is constantly converted to a SciPy Rotation and then to a matrix, Euler angles or another quaternion.
• Now the starting ._theta angle for Camera is -TAU/4 instead of 0, which is more representative of its initial orientation. When increasing ._phi a bit, the Camera is positioned over the negative Y axis, which corresponds to theta=-TAU/4, not theta=0.
• Camera now directly uses .focal_distance instead of .focal_dist_to_height. This was needed in order to correctly implement zooming.
• Now Camera can be properly zoomed in and out. Previously, it couldn't. Scaling the Camera used to only cause a difference in the focal distance.

Reviewer Checklist

• The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• If applicable: newly added functions and classes are tested

[JasonGrace2282]

LGTM! Just one small change/discussion

[JasonGrace2282]

I'm not a big fan of this in experimental, I would prefer a hard error (XYZ cannot be rendered)

Maybe a marker class is neccessary? Something like

class InvisibleMobject:
    pass

class Camera(OpenGLMobject, InvisibleMobject):
    ...