This uses a pretty slow path with readback to the CPU. It should be
possible to use SV_Depth in HLSL to resolve without a readback, but
that will be left for a future optimization.

Enables the WebGL 2 tests gles3/fbomultisample and fboinvalidate/sub.

BUG=angleproject:1246

Change-Id: Id67178b0f6374cf53e4e107428637546ecca4124
Reviewed-on: https://chromium-review.googlesource.com/359956Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>