Use TextureArray to store MorphTargets

[mvaligursky]

Currently each MorphTarget is stored in a texture. To blend more targets, we create (cached) shader based on the number of active weights, with up to 16 (based on the device caps) textures. If more weights are active, we multi pass.

Now that we don't need to support WebGL1, we should put all of the data to a single texture array, allowing them to be all available in the shader. No more multipass handling. No more multiple shaders - have a single shader and loop over active weights and sample from the texture array.