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/+/1610188Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>