Vulkan: Add support for surface multisampling A multisample image is created for the surface if multisampling is enabled. Prior to present, this multisample image is resolved into the swapchain image. FramebufferVk::readPixelsImpl similarly has got the ability to resolve the region of interest into a temporary image prior to readback. Tests are added to render a point, line and a triangle on a 4x multisampled surface. Bug: angleproject:3204 Change-Id: I34aca502fa1918b5cbf000ff11521c350372e051 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1610188 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>