- XFB is currently emulated by writing to storage buffers.

- Metal doesn't allow vertex shader to both write to storage buffers and
  to stage output (i.e clip position). So if GL_RASTERIZER_DISCARD is
  NOT enabled, the draw with XFB enabled will have 2 passes:
  + First pass: vertex shader writes to XFB buffers + not write to stage
    output + disable rasterizer.
  + Second pass: vertex shader writes to stage output (i.e.
    [[position]]) + enable rasterizer. If GL_RASTERIZER_DISCARD is
    enabled, the second pass is omitted.
  + This effectively executes the same vertex shader twice. TODO:
    possible improvement is writing vertex outputs to buffer in first
    pass then re-use that buffer as input for second pass which has a
    passthrough vertex shader.

- If GL_RASTERIZER_DISCARD is enabled, and XFB is enabled:
  + Only first pass above will be executed, and the render pass will use
    an empty 1x1 texture attachment since rasterization is not needed.

- If GL_RASTERIZER_DISCARD is enabled, but XFB is NOT enabled:
  + we still enable Metal rasterizer.
  + but vertex shader must emulate the discard by writing gl_Position =
    (-3, -3, -3, 1). This effectively moves the vertex out of clip
    space's visible area.
  + This is because GLSL still allows vertex shader to write to stage
    output when rasterizer is disabled. However, Metal doesn't allow
    that. In Metal, if rasterizer is disabled, then vertex shader must
    not write to stage output.

- See src/libANGLE/renderer/metal/doc/TransformFeedback.md for more
  details.

Bug: angleproject:2634
Change-Id: I6c700e031052560326b7f660ee7597202d38e6aa
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2408594Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>