For textures that use this extension, a multisampled texture is
implicitly created for the texture.
Upon write or read, the multisampled texture is either return
to be drawn to or resolved and returned as a single sampled texture.

This is the functionality change with end2end tests.

Bug: angleproject:980428
Change-Id: I5776875a132fed7a3f4f00fb02f9e8e250684630
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1773717
Commit-Queue: Rafael Cintron <rafael.cintron@microsoft.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>