Start implementing glClear Currently, because both SurfaceWgpu::getAttachmentRenderTarget() and TextureWgpu::getAttachmentRenderTarget() are unimplemented, there is no actual RenderTargetWgpu in the RenderTargetCache to clear. Bug: angleproject:8582 Change-Id: I9ad33c57d533d81178d7d2a802d35b106ece5848 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5388076 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Liza Burakova <liza@chromium.org> Commit-Queue: Matthew Denton <mpdenton@chromium.org>