Metal: Implement EXT_occlusion_query_boolean. - Metal's occlusion(called visibility) query operates per render pass. Implementation details are in src/libANGLE/renderer/metal/doc/OcclusionQueries.md. - New tests: - OcclusionQueriesTest.ClearNotCounted. - OcclusionQueriesTest.MultiQueries (failure on OpenGL/D3D11 back-end). Bug: angleproject:2634 Change-Id: Idd1327b5472d0e8c2b69307a7f04a1da4a847a40 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2336121 Commit-Queue: Le Hoang Quyen <le.hoang.q@gmail.com> Reviewed-by: back sept 10 - Jamie Madill <jmadill@chromium.org> Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>