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Summary: Fixes GitHub issue #751. The vectorized implementation of bilinear interpolation didn't properly handle the edge cases in the same way as the `grid_sample` method in PyTorch.

Reviewed By: bottler

Differential Revision: D30684208

fbshipit-source-id: edf241ecbd72d46b94ad340a4e601e26c83db88e

Summary:
A fairly big refactor of the texturing API with some breaking changes to how textures are defined.

Main changes:
- There are now 3 types of texture classes: `TexturesUV`, `TexturesAtlas` and `TexturesVertex`. Each class:
   - has a `sample_textures` function which accepts the `fragments` from rasterization and returns `texels`. This means that the shaders will not need to know the type of the mesh texture which will resolve several issues people were reporting on GitHub.
  -  has a `join_batch` method for joining multiple textures of the same type into a batch

Reviewed By: gkioxari

Differential Revision: D21067427

fbshipit-source-id: 4b346500a60181e72fdd1b0dd89b5505c7a33926