The GLTF yfov value is the vertical field of view. The camera.fov
attribute in THREE.js is also a vertical field of view, so it should not
be divided by the aspect ratio when exporting.

- Adds AnimationUtils.insertKeyframe().
- Adds KeyframeTrack.clone().
- Enables GLTFExporter to automatically merge animation tracks affecting
individual morph targets.

Updated processImage cache scheme to handle different combinations of mimeType and flipY export options.

This locks the function down to process one specific image and prevents use of texture properties after cache retrieval that would violate the cache scheme.

Reduces output glb size considerably when the scene contains shared images and embedImages is enabled for the export.