Add `-e webp` option to mesh_pipeline.
Add `-e webp` option to mesh_pipeline.

The mesh_pipeline, by default, now uses the original
texture file extension in material files.

We use `-e webp` to tell mesh_pipeline that all the
texture files will be converted to webp, and therefore
the material files should have the .webp extension.

This is new behavior in the mesh_pipeline, which previously
always assumed the material textures would be .webp.

Tested: Deleted and remade all meshes in Zooshi. Runs fine.
Change-Id: I875609b4b27e5d0dbd33db64299b00fbb7716b1d

google/zooshi,google/zooshi,google/zooshi,google/zooshi,google/zooshi