Add RenderTargetWgpu
Based on RenderTargetMtl without any multisampling support for now.
Bug: angleproject:8582
Change-Id: I4c77e747b341f08a0dd88f507e6d805d34a01d24
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5386899
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Liza Burakova <liza@chromium.org>
Commit-Queue: Matthew Denton <mpdenton@chromium.org>
