There are bugs in Apple's OpenGL driver when uploading to textures of
these formats via pixel buffer objects.

Because automatic graphics switching might activate the AMD GPU at any
time, use this workaround on dual-GPU MacBook Pros even if the Intel
GPU is active.

Fixes the following WebGL 2.0 conformance tests:

  deqp/functional/gles3/texturespecification/
    teximage2d_pbo_2d_00.html
    teximage2d_pbo_2d_01.html
    texsubimage2d_pbo_2d_00.html
    texsubimage2d_pbo_2d_01.html
    texsubimage2d_pbo_cube_01.html

Bug: angleproject:5469
Change-Id: I7d65dc5f664159b4b59d46400a8d9c492f67040b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2595245
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>