Add support for vec3 vertex-shaded objects

[Blayzeing]

Currently all vertex-shaded objects are assumed to be vec4, and objects not so have their vertex shading ignored.

This is processed near the bottom of the processGLTFNode method in MulticamScene.cpp.

[Blayzeing]

Also see the TODO below the TODO for this that suggests requiring a "vertex-colours" extras property to be true if an object's going to use vertex colours.

[optseb]

See Pull request #32