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Summary:
If you join several meshes which have TexturesUV textures using join_meshes_as_scene then we amalgamate all the texture images in to a single one. This now checks if some of the images are equal (i.e. the tensors are the same tensor, in the `is` sense; they have the same `id` in Python) and only uses one copy if they are.

I have an example of a massive scene made of several textured meshes with some shared, where this makes the difference between fitting the data on the GPU and not.

Reviewed By: theschnitz

Differential Revision: D25982364

fbshipit-source-id: a8228805f38475c796302e27328a340d9b56c8ef