Optimize picking

The fix for #583 slows down the fragment shader for picking because it executes the full fragment shader for materials. If we know a material doesn't have any transparent parts (and doesn't write depth in the future), we can just use a simple pass-through shader.

Start by tracing the picking path and fragment shader behavior changed by #583; the issue names no files or tests. Identify where material transparency and future depth-writing are known, then verify that opaque materials use the simpler picking path while transparent materials retain the existing behavior.

Written by the indexing model from the issue text.