Add GLSL1 and GLSL3 version constants

Addressing PR feedback from https://github.com/mrdoob/three.js/pull/18822#pullrequestreview-386940541 and https://github.com/mrdoob/three.js/pull/18822#issuecomment-608196773

- Updated blend mode constant names to 'NormalAnimationBlendMode' and 'AdditiveAnimationBlendMode'
- Removed clone options from AnimationUtils.makeClipAdditive
- Added blendMode as a serialized property to AnimationClip, defaulting to NormalAnimationBlendMode
- Added reference clip input to AnimationUtils.makeClipAdditive and set blendMode on the target clip to AdditiveAnimationBlendMode
- Updated example script to not pass AnimationBlendMode to mixer.clipAction since the property gets set during makeClipAdditive

- Added AnimationBlendMode constants: OverrideBlendMode and AdditiveBlendMode
- Updated AnimationAction and AnimationMixer to use blendMode rather than isAdditive boolean
- Updated example to use AnimationBlendMode constants rather than isAdditive boolean

This change allows transcoding new UASTC .basis files to BPTC which
results in much higher quality compared to UASTC -> BC3 transcoding.

The change needs to introduce BPTC support to three.js core for this to
work; the format is introduced in EXT_texture_compression_bptc which is
available in Chrome Canary on Windows (and possibly elsewhere?)

The update to Basis transcoder just pulls it directly from Binomial
repository; the transcoder is a bit larger now, unfortunately, because
it needs to support UASTC.

Note that we use mode 5 unconditionally to make sure RGB and RGBA
textures work.

BPTC (BC7) and BC3 are the same size in memory so this shouldn't
negatively impact VRAM consumption.