[Feat] (DirectVulkan, MG_Impl): occlusion queries via Vulkan query pools

GL_SAMPLES_PASSED / GL_ANY_SAMPLES_PASSED(_CONSERVATIVE) now work: every
app draw between Begin/EndQuery is wrapped in a slot of a host-reset
occlusion query pool (precise counts when occlusionQueryPrecise is
granted), and the result flush ends any active render pass before
submitting, waits, sums the slots and recycles them. ANY_* targets
report the boolean form; GL_QUERY_COUNTER_BITS and GL_CURRENT_QUERY
answer for the occlusion targets, and deleting an active query releases
its slot. Draw-time depth/stencil state also honors attachment absence:
a framebuffer without a depth (stencil) attachment behaves as if that
test always passes, even when a packed depth-stencil image is attached
through only one half (verify_partial_attachments.*).

MobileGL/MG_Backend/BackendObject.h

@@ -220,6 +220,11 @@ namespace MobileGL {
             // and leave the query readable later.
             Bool (*GetQueryResult64)(BackendQueryHandle query, Bool wait, Uint64* outNanoseconds);
             void (*DeleteBackendQuery)(BackendQueryHandle query);
+            // GL_SAMPLES_PASSED occlusion queries (optional; null = unsupported,
+            // the frontend then rejects the target). Results/deletion flow through
+            // GetQueryResult64 / DeleteBackendQuery like timer queries.
+            BackendQueryHandle (*BeginOcclusionQuery)();
+            void (*EndOcclusionQuery)(BackendQueryHandle query);
             Int64 (*GetGpuTimestampNs)(); // glGetInteger64v(GL_TIMESTAMP); 0 if unsupported
         };
         struct GlobalBackendFunctionsTable {