Add normal compression to Primitive

[pjcozzi]

There's a new JCGT paper: A Survey of Efficient Representations
for Independent Unit Vectors. This is an excellent survey of space/time/quality tradeoffs for compressed normals.

To save memory, let's add this to the primitive's geometry pipeline. Read the paper to see what the best tradeoffs are for us, but this could be as simple as patching our vertex shaders with hemioct_to_float32x3 from Listing 6 and adding float32x3_to_hemioct in Listing 6 to our geometry pipeline as early as possible to minimize memory copies (do not move this into individual geometries though).

Let's do two stages:

• Add [options.compressNormals=true] or whatever flag to the Primitive constructor to compress normal, binormal, and tangent (when they are present). Patch the vertex shader.
• Since normal attributes will be vec2 now, when present pack normal and st into a vec4. Pack binormal and tangent into a vec4. Patch the vertex shader.