D3D11: Handle Clear* commands for layered framebuffers According to the ANGLE_multiview spec, Clear* commands only affect the range of attached layers to the multi-view layered framebuffer. The patch extends ImageIndex so that the range of attached layers is tracked and a render target view can be created with that range of texture array slices attached. The special case of scissored clears for depth and stencil attachments is handled by instancing the same number of quads as there are views and selecting the layer within a geometry shader. BUG=angleproject:2062 TEST=angle_end2end_tests Change-Id: Ibea248b980513f83d918652030a72c62c7ecd88b Reviewed-on: https://chromium-review.googlesource.com/632256 Commit-Queue: Martin Radev <mradev@nvidia.com> Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>