Add minimal run as service option to sample

New flag service causes the application to run as a "service" main
overlay application.

overlay-sample/overlay_sample.cpp

@@ -85,6 +85,7 @@ constexpr int gFudgeMinAlpha = 128;
 
 int gLayerPlacement = 0;
 float gLayerRotationalOffset = 0.f;
+bool gEmptyFrame = false;
 
 // OpenXR will give us a LUID.  This function will walk adapters to find
 // the adapter matching that LUID, then create an ID3D11Device* from it so
@@ -420,8 +421,8 @@ void OpenXRProgram::CreateSession(ID3D11Device* d3d11Device, bool requestOverlay
     d3dBinding.device = d3d11Device;
     chain = &d3dBinding;
 
+    XrSessionCreateInfoOverlayEXTX sessionCreateInfoOverlay{ XR_TYPE_SESSION_CREATE_INFO_OVERLAY_EXTX };
     if(requestOverlaySession) {
-        XrSessionCreateInfoOverlayEXTX sessionCreateInfoOverlay{ XR_TYPE_SESSION_CREATE_INFO_OVERLAY_EXTX };
         sessionCreateInfoOverlay.next = chain;
         sessionCreateInfoOverlay.createFlags = 0;
         sessionCreateInfoOverlay.sessionLayersPlacement = gLayerPlacement;
@@ -1037,6 +1038,7 @@ void usage(const char *programName)
     std::cerr << "usage: overlay-sample [options]\n";
     std::cerr << "options:\n";
     std::cerr << "    --main                 Create a main session, not an overlay session\n";
+    std::cerr << "    --service              Create a main session with no content\n";
     std::cerr << "    --placement N          Set overlay layer level to N    [default 0]\n";
     std::cerr << "    --rotational-offset N  Angle in radians to offset the layer clockwise about\n";
     std::cerr << "                           the stage space world up vector [default 0]\n";
@@ -1058,6 +1060,10 @@ int main( int argc, char **argv )
         } else if (strcmp(argv[arg], "--main") == 0) {
             createOverlaySession = false;
             arg += 1;
+        } else if (strcmp(argv[arg], "--service") == 0) {
+            createOverlaySession = false;
+            gEmptyFrame = true;
+            arg += 1;
 
         } else if ((strcmp(argv[arg], "--rotational-offset") == 0) || (strcmp(argv[arg], "--rot") == 0)) {
             if (arg + 1 >= argc) {
@@ -1279,7 +1285,7 @@ int main( int argc, char **argv )
 
                 program.BeginFrame();
 
-                if(shouldRender) {
+                if(shouldRender && !gEmptyFrame) {
 
 
                     for(int eye = 0; eye < 2; eye++) {