This is needed for antialiased multiview rendering.

The extension is written against OpenGL ES 3.1. To add support for it
in OpenGL ES 3.0, an extension for exposing non-array multisample
textures will also be required (ANGLE_texture_multisample). This has
been previously proposed though not yet submitted to ANGLE.

The extension spec provides robustness guarantees for sampling
multisample textures. It still needs to be tested whether native
implementations follow these guarantees or if the spec needs to be
adjusted to allow for clamping coordinates. The native
KHR_robust_buffer_access_behavior is not very explicit about whether
the texel fetch robustness applies also to multisample textures.

BUG=angleproject:2775

Change-Id: I1bd721008a10bee29f8e2e06edca19370a9932a7
Reviewed-on: https://chromium-review.googlesource.com/1186336Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>