InstancedMesh: Add support for morph targets.

[wizgrav]

@Mugen87 I took a stub at adding full morph target support as I told you on the other PR and the results were surprisingly good. I don't notice any performance difference with the horse model which has 15 targets so it seems quite performant.

Now everything is loaded on a texture including the base influence so it will support every case and the API is identical to the attribute approach. I left the example as is and will make some more to test other cases but wont push in this PR

demo here (you may need to refresh cache to be sure, output is identical to the previous one)

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

Filesize dev	Filesize PR	Diff
674.2 kB (167.5 kB)	675.9 kB (167.9 kB)	+1.7 kB

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

Filesize dev	Filesize PR	Diff
455.1 kB (110.5 kB)	456.4 kB (110.7 kB)	+1.36 kB

[Mugen87]

The solution looks awesome now!

[wizgrav]

@Mugen87 I wrote an example using the face and it's slower than the horse but that was not unexpected. I havent pushed it in the PR or even added it in the examples list but you can check it out directly here

[Mugen87]

Regarding the code complexity in WebGLRenderer: Yes, this PR adds additional complexity to the renderer but also adds a useful feature. When WebGL 1 support is removed next month, the renderer can be simplified at certain places. E.g. the entire WebGL 1 code path for morph targets can be deleted. That should keep the morph target related code manageable even with this PR in place.