This is split off from a change to implement multisampled textures for
the Vulkan back-end, and will come before that change.  The changes
include:

- Make a common utility rx::GetSamplePosition() function.  D3D11 and
  Vulkan use the same standard sample positions/locations for 1, 2, 4,
  8, and 16 samples.  The D3D11 back-end has a utility function for
  this, which is being moved to a common location--for use by both the
  D3D11 and Vulkan back-ends.

- Texture::setStorageMultisample() handles converting the "requested number of
  samples" to the actual number of samples used (e.g. converting 3 to 4),
  supported by the underlying back-end).  The actual number used is stored in
  gl::TextureState::mImageDescs, for use by other GLES commands.

- Change some end2end tests to not make assumptions about the supported number
  of samples, but to properly query what is supported.

Bug: angleproject:3565
Bug: angleproject:4196
Change-Id: I1dc12fedd0f8fb4975f90d87486e443b069b7141
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1948535
Commit-Queue: Ian Elliott <ianelliott@google.com>
Reviewed-by: Ian Elliott <ianelliott@google.com>