[Feat] (DirectVulkan, MG_Impl): GPU transform feedback primitive queries

The TF primitive queries now ride VK_QUERY_TYPE_TRANSFORM_FEEDBACK_STREAM_EXT
pools when the device reports transformFeedbackQueries: each captured draw
is wrapped in a slot (shared between both GL targets when active
together), and results sum the (written, needed) pairs -
GL_TRANSFORM_FEEDBACK_PRIMITIVES_WRITTEN from the first,
GL_PRIMITIVES_GENERATED from the second. This is exact through geometry
shaders, so KHR-GL33.transform_feedback.query_geometry_* pass; the CPU
accounting delta remains the fallback for backends without the feature.

MobileGL/MG_Backend/BackendObject.h

@@ -225,6 +225,10 @@ namespace MobileGL {
             // GetQueryResult64 / DeleteBackendQuery like timer queries.
             BackendQueryHandle (*BeginOcclusionQuery)();
             void (*EndOcclusionQuery)(BackendQueryHandle query);
+            // Transform feedback primitive queries backed by real GPU query pools
+            // (optional; null = frontend falls back to CPU accounting).
+            BackendQueryHandle (*BeginXfbPrimitivesQuery)(Bool generated);
+            void (*EndXfbPrimitivesQuery)(BackendQueryHandle query);
             Int64 (*GetGpuTimestampNs)(); // glGetInteger64v(GL_TIMESTAMP); 0 if unsupported
         };
         struct GlobalBackendFunctionsTable {