Docs: Math updates
Preserve face ordering in OBJLoader and OBJMTLLoader.

OBJ loaders did not correctly preserve vertex and face list ordering and were creating duplicate vertex entries, rather than letting faces share vertices.
Generalize PLYLoader to correctly handle ascii ply files with unrecognized components.

Now won’t give mangled results if the ply file is saved with colors or normals. Included is a test file that is saved with color and broke the old loader.
PLYLoader: Add support for color in ascii ply files.
PlyLoader: Added support for binary ply files.
Updated Vector4's setAxisAngleFromRotationMatrix and setAxisAngleFromQuaternion docs
Blender exporter taking flipyz into account
Blender exporter normal bug fix

Global rotation is not taken into account for normals in loader.
This leads to all objects/cameras/lights in the scene being rotated correctly,
but with normals pointing into the wrong direction.

Not applying rotation at all fixes this problem
OBJLoader & OBJMTLLoader: Fixed sharing of Vector2 & Vector3 objects by UVs & normals.

Should bring the OBJ(MTL)Loader into compliance with #3762.
OBJ{MTL}Loader: Formatting & whitespace fixes.
PLYLoader: Formatting & whitespace fixes.
Add start and end events triggered on mouse/wheel/touch down/up for trackball control

The start and end events can be used to start or stop an animation loop, for example, so that the animation loop does not have to run continuously.
More math doc updates
Merge branch 'documentation_updates4' into documentation_updates3
Merge remote-tracking branch 'upstream/dev' into documentation_updates3
More verbose description for opacity/transparent
Merge branch 'dev' of https://github.com/mrdoob/three.js into dev
Merge branch 'plyloader-improvements' of https://github.com/algrs/three.js into dev
Merge branch 'objloader-preserve-face-order' of https://github.com/algrs/three.js into dev
Merge branch 'dev' of https://github.com/mrdoob/three.js into dev