Implement render-first pacing for Vulkan backend and enhance view location handling

runtime/include/vr/openxr_runtime.h

@@ -153,6 +153,11 @@ public:
     OpenXRFrameStatus WaitFrame(OpenXRFrame& frame);
     bool BeginFrame(const OpenXRFrame& frame);
     bool LocateViews(OpenXRFrame& frame);
+    // Locates the views for `display_time` outside the frame protocol, for a
+    // packet whose eyes are rendered before the compositor frame that will show
+    // them is begun (the standalone backend's render-first pacing). Fills the
+    // frame's display time, views, flags and validity; requires a running session.
+    bool LocateViewsAt(XrTime display_time, OpenXRFrame& frame);
     bool EndFrame(
         const OpenXRFrame& frame,
         const XrCompositionLayerBaseHeader* const* layers,
@@ -224,6 +229,7 @@ private:
     bool EnumerateSwapchainFormats();
     bool HandleSessionStateChanged(const XrEventDataSessionStateChanged& event);
     bool IsFrameTokenCurrent(const OpenXRFrame& frame, FramePhase expected) const;
+    bool LocateViewsForFrame(OpenXRFrame& frame);
     void ResetFrameState();
     void DestroyReferenceSpaces();
     void ResetSessionState();

runtime/include/vr/openxr_vulkan.h

@@ -58,6 +58,20 @@ public:
     bool RepeatFrame(const OpenXRBackendFrame& frame);
     bool FinishFrame(OpenXRBackendFrame& frame, bool submit_layer);
 
+    // Render-first pacing, used while VR interpolation is off. A packet is prepared with the
+    // views located for an estimated display time and handed to Aurora without a compositor
+    // frame open; once Aurora has rendered the eyes into the shared buffers, the compositor frame
+    // is begun, the eyes are copied into its swapchain images and it is ended at once. A headset
+    // frame therefore never waits for a game frame: it stays open for the copy alone.
+    OpenXRBeginStatus PreparePacket(const OpenXRPresentation& presentation, OpenXRBackendFrame& packet);
+    bool TryCancelPendingPacket(OpenXRBackendFrame& packet);
+    OpenXRBeginStatus BeginFrameForPacket(const OpenXRBackendFrame& packet, OpenXRBackendFrame& frame);
+    OpenXRSubmissionStatus CopyRenderedEyes(const OpenXRBackendFrame& frame);
+    // One compositor cycle that resubmits the retained layer (or nothing), with no frame left
+    // active: keeps the runtime fed while the eyes are still being rendered and learns the
+    // display timing the next packet is located for.
+    OpenXRBeginStatus KeepAliveCycle();
+
     // Drains this backend's own queue before tearing down. Returns false only
     // when the private device could not be waited on, in which case the caller
     // retains the backend and runtime for the process lifetime.