Do not render VR mirror texture is the headset is not rendering

DX11 will signal an error on the mirror present if the compositor
is not serving the application frames to the hardware.

src/hmd_ovr.cpp

@@ -197,7 +197,10 @@ namespace bgfx
 		}
 
 		// perform mirror texture blit right after the entire frame is submitted to HMD
-		m_mirror->blit(m_hmd);
+		if (result != ovrSuccess_NotVisible)
+		{
+			m_mirror->blit(m_hmd);
+		}
 
 		m_hmdToEyeOffset[0] = m_erd[0].HmdToEyeOffset;
 		m_hmdToEyeOffset[1] = m_erd[1].HmdToEyeOffset;