This portability feature allows a single T-Rex capture done on Vulkan
to work seamlessly on the ANGLE GL back-end and the WGL-loaded NVIDIA
OpenGL driver.

Previously the uniform locations would not be consistent between ANGLE
and a native driver. And incompatiblities would trigger error messages
between ANGLE back-ends.

Will allow us to compare ANGLE performance vs the native driver using a
capture.

Example captured calls:

    glLinkProgram(gShaderProgramMap[3]);
    UpdateUniformLocation(gShaderProgramMap[3], "s_texture", 0);

Bug: angleproject:4411
Change-Id: I5ddc76d75e15a9291b351e2aab94af4881f6ef47
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2068121
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>