Tutorials textures updates and fix bug in extending meshes with uv textures

Summary:
Found a bug in extending textures with vertex uv coordinates. This was due to the padded -> list conversion of vertex uv coordinates i.e.                 The number of vertices in the mesh and in verts_uvs can differ
e.g. if a vertex is shared between 3 faces, it can
have up to 3 different uv coordinates. Therefore we cannot convert directly from padded to list using _num_verts_per_mesh

Reviewed By: bottler

Differential Revision: D23233595

fbshipit-source-id: 0c66d15baae697ead0bdc384f74c27d4c6539fc9

Author: nikhilaravi