D3D11: Handle Clear* commands for side-by-side framebuffers The patch modifies Clear11 to add support for clearing the color, depth and stencil attachments to a side-by-side framebuffer: - Color attachments are cleared through the command ClearView. - Depth and stencil attachments are cleared by drawing a quad on top of each view. BUG=angleproject:2062 TEST=angle_end2end_tests Change-Id: I5753a72222216a48cd954eb1219bc58f968735fc Reviewed-on: https://chromium-review.googlesource.com/603853 Commit-Queue: Martin Radev <mradev@nvidia.com> Reviewed-by: Geoff Lang <geofflang@chromium.org>