OpenGL tests all fail on macOS

[alexreinking]

While the OpenGL tests build on macOS, they do not run. They all crash with

Error: (at /Users/halidenightly/build_bot_new/worker/mac-64-1000/halide/test/opengl/varying.cpp:62)
The pipeline returned exit status -1 but halide_error was never called.

With the line of the error suitably replaced for the test.

[abadams]

I think we need to fix openglcompute and get it widely tested and just delete the opengl backend. The current opengl tests should be cherry-picked into openglcompute tests where appropriate.

[alexreinking]

That would be good, but can we not let it block #4644?

[abadams]

Sure, but why would it? That PR doesn't turn on testing for opengl, does it?

[alexreinking]

Sorry, see halide/build_bot#60