This functionality is exercised by running angle_deqp_gles31_tests with the
following sets of command arguments:

  --gtest_filter=dEQP.GLES31/functional_texture_multisample* --use-angle=vulkan

  --gtest_filter=dEQP.GLES31/functional_state_query_texture_level_texture_2d_multisample* --use-angle=vulkan

  --gtest_filter=dEQP.GLES31/functional_state_query_texture_texture_2d_multisample_texture_immutable* --use-angle=vulkan

The following are some high-level design notes:

- 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.

- ANGLE uses the Vulkan standard sample locations/positions.  If the underlying
  Vulkan driver's VkPhysicalDeviceLimits::standardSampleLocations is false,
  ANGLE limits itself to GLES 2.0 (i.e. before GLES 3.0 which adds multisample
  renderbuffers).

- The Vulkan specification currently doesn't support ANGLE providing support
  for GLES 1-sample textures, because of the following Vulkan specification
  statement:

  - If the image was created with VkImageCreateInfo::samples equal to
    VK_SAMPLE_COUNT_1_BIT, the instruction must: have MS = 0.

- At least one Vulkan driver returns different
  VkPhysicalDeviceLimits::*SampleCounts for different formats.  Because of
  this, ANGLE does a logical-AND of all values in order to only support the
  commonly-available numbers of samples.

The detailed design document is located at:
https://docs.google.com/document/d/1NiM8gAR74iGGXGTE6IP1ChdDUZjhtXRuJdtEp_wGFEM/edit?usp=sharing

Bug: angleproject:3565
Bug: angleproject:4103
Bug: angleproject:4104
Bug: angleproject:4196
Bug: angleproject:4197
Bug: angleproject:4198
Change-Id: I28921badf9568427799b0af347198b5df06c2db0
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1919982
Commit-Queue: Ian Elliott <ianelliott@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>