* 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.
* On some GPUs the stencil output comes in all channels, not just green, and
it's unclear if this is legal or not. To be safe just mask out the green we
expect.
* 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.
baldurk