[Fix] (Remote): the present record presents a host-framed surface

A split client's swap reaches the server as a present record, and ServerVerbSink::OnPresent only
ran the backend's Present - an eglSwapBuffers of the pbuffer the context was made with.
BackendObject::SwapEGLBuffers is the only route to PresentHostFrame and a split client never takes
it, so over TCP the display host was never told a frame was drawn: the app offered thousands of
frames and presented none.

BackendObject::PresentHostFrameIfCurrent presents the host frame when the current surface is
host-framed and is a no-op otherwise; OnPresent calls it after Present and before the credit goes
back, so a client released by the credit is released by a frame that is on its way to the glass.

MobileGL/MG_Backend/BackendObject.cpp

@@ -479,6 +479,12 @@ namespace MobileGL::MG_Backend {
         return false;
     }
 
+    Bool BackendObject::PresentHostFrameIfCurrent() {
+        const std::lock_guard<std::recursive_mutex> lock(m_eglStateMutex);
+        if (!m_eglSurfaceInitialized || m_eglSurfaceKind != SurfaceKind::HostFrame) return true;
+        return PresentHostFrame();
+    }
+
     Bool BackendObject::IsEGLSurfaceCurrent(EGLSurface surface) const {
         if (surface == EGL_NO_SURFACE) {
             return false;

MobileGL/MG_Backend/BackendObject.h

@@ -602,6 +602,11 @@ namespace MobileGL {
             virtual Bool CreateEGLHostFrameSurface(EGLSurface surface, Uint32 width, Uint32 height);
             virtual Bool InitHostFrameSurface(EGLint width, EGLint height);
             virtual Bool PresentHostFrame();
+            // The present RECORD's half of a host-framed swap.  A split client's swap reaches the
+            // server as a present record (ServerVerbSink::OnPresent), never as SwapEGLBuffers, so
+            // without this the host was never told a frame was drawn.  True when the current
+            // surface is not host-framed: there is nothing to present then.
+            Bool PresentHostFrameIfCurrent();
             virtual Bool MakeEGLCurrent(EGLDisplay dpy, EGLSurface draw, EGLSurface read, EGLContext ctx);
             virtual Bool SwapEGLBuffers(EGLDisplay dpy, EGLSurface draw);
             // Forwards the app-requested eglSwapInterval to the backend's native

MobileGL/MG_Remote/Server/PipeApplier.cpp

@@ -404,6 +404,15 @@ namespace MobileGL::MG_Remote::Server {
         // was wanted, recorded here so a future swap-without-present is looked for rather than
         // assumed impossible. Presents() is exposed for a lane that wants to make the comparison.
         table->Present();
+        // A HOST-FRAMED SURFACE IS PRESENTED BY TELLING THE HOST, not by the swap above (which
+        // swaps the pbuffer the context was made with).  BackendObject::SwapEGLBuffers is the
+        // only other route to PresentHostFrame and a split client never reaches it, so this is
+        // where the display host learns the frame is drawn - before the credit goes back, so a
+        // client released by it is released by a frame that is on its way to the glass.
+        if (m_backend != nullptr) {
+            if (!m_backend->PresentHostFrameIfCurrent())
+                MGLOG_E_ONCE("OnPresent: the host frame was not presented (PresentHostFrame failed; see above)");
+        }
         ++m_presents;
         // FrameSerial 0 means "the server stamps its own" (c1-v1 8.3): P5 has no client-side
         // present credit, so the client sends 0 and the frame count on this side IS the serial.