* It's not particularly scalable and can be brittle to driver changes, and we
can use targeted specific pixel tests to check what we really want - to see if
the output has rendered correctly.
* Overlay tests still check files directly - this is a future refactor to
remove.
* This is needed particularly for integer textures which are incomplete if
sampled (even with a non-sampling instruction) with the default LINEAR
filtering.
* These tests ensure that texture rendering works correctly for all different
types of texture types, and for all formats, across different APIs, including
across a remote-proxy connection.