Testbed doesn't support Apple's Metal API

[louis-langholtz]

Expected/Desired Behavior or Experience:

Testbed uses Apple's Metal on supporting platforms.

Actual Behavior:

Only OpenGL is currently supported.

Related Info:

• https://en.wikipedia.org/wiki/Metal_(API)
• https://www.raywenderlich.com/9211-moving-from-opengl-to-metal

[louis-langholtz]

Kind of leaning against doing this enhancement if the one described in issue #367 can be done.