Some WebGL2 internal formats rely on new WebGL2 formats. This change
add the 4 new PixelFormat constants that are needed

* Adds `internalFormat` attribute to Texture.js and Texture.d.ts
* Adds `internalFormat` string mapping to GL constant in WebGLTextures.js
* Adds `internalFormat` in Texture documentation
* Adds internal format string constants to constants documentation

The call to generateMipmap() at the bottom of uploadTexture() was previously assuming _gl.TEXTURE_2D as the texture type, which broke automatic mipmapping for other texture types.

To avoid creating new Array objects, just change the length to 0; we
can't index into the array anyhow.

When using a material that has morph target support, we need to supply
morph target weights, and - crucially, as of r111 - base target weight.

Before r111, using a material with morph targets and an object without
morph target influences resulted in us not setting up morph weights
which in most cases would result in rendering the object normally.

After r111, in this situation we'd accidentally use default (0) base
influence which resulted in object not rendering at all.

This change now calls morph target update if the material needs morph
target setup; the update method sets up dummy weights (1 base and 0
for each target) to make sure the shader outputs the value of the base
attribute only.

Related to https://github.com/mrdoob/three.js/issues/17642

This reverts commit 481ac66c.