MAINT-7977: Additional cleanup per code reviews.

Introduce helper classes to manage paired initProfile() / finishProfile()
calls and gBenchmarkProgram.bind() / unbind() calls.

Make TextureHolder a class instead of a struct.

Per Henri Beauchamp, since gpu_benchmark() takes a very early exit if
(!gGLManager.mHasTimerQuery), subsequent tests of mHasTimerQuery are
redundant. Remove.

One of those tests controls the busted_finish bool, which can never become
true. Remove that and all tests on it.