- Max point size is set to 64, since this is the max size observed. It
  is not 511 as
  https://developer.apple.com/metal/Metal-Feature-Set-Tables.pdf claims.
- RGB565 is emulated on macOS by RGBA, needs to disable alpha write for
  this format. This was already done for TextureMtl, but wasn't for
  RenderBufferMtl.
- Also enable true non-power-of-two textures support (OES_texture_npot).
  This was technically already supported, just that the extension wasn't
  advertised yet.

- New Test: FramebufferFormatsTest.RGB565Renderbuffer

Bug: angleproject:4816
Bug: angleproject:2634
Change-Id: Ie7e3efb4cb29bb9a3fd706c26e2b50b42ff3f6be
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2281797
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>