Vulkan: Fix MSRTT w.r.t stencil-only unresolve The stencil bit was being silently masked out in a bitset in FramebufferDesc that tracked whether the framebuffer requires any unresolve. If only stencil needs unresolving, this mask was zero, leading to an incorrect framebuffer getting pulled from the cache. A follow up change will add an ASSERT in BitSetT to catch such errors in the future. This issue was only reproducible on SwiftShader and AMD as the only implementers of VK_EXT_shader_stencil_export. Bug: angleproject:6840 Bug: angleproject:6324 Change-Id: I4f055982ebd75f621ec1e34b0d60eaa497c27b17 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3349979 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>