add new button handler for system button on the left controller

- when used in gzdoomvr mode will behave like Backspace
- when used in questzdoom mode will behave like Left Ctrl

src/rendering/gl/stereo3d/gl_openvr.cpp

@@ -1570,6 +1570,9 @@ namespace s3d
 
 			// k_EButton_Axis4 (unknown if used by any controller at all)
 			HandleVRButton(lastState, newState, openvr::vr::k_EButton_Axis4, KEY_JOY3, role * (KEY_JOY4 - KEY_JOY3));
+
+			// k_EButton_System
+			HandleVRButton(lastState, newState, openvr::vr::k_EButton_System, KEY_BACKSPACE, role);
 		}
 
 		lastState = newState;
@@ -1836,6 +1839,12 @@ namespace s3d
 		static int joy_mode = vr_joy_mode;
 		if (joy_mode == 1) 
 		{
+			//Menu button - invoke menu
+			Joy_GenerateButtonEvents(
+				((pSecondaryTrackedRemoteOld->ulButtonPressed & ButtonMaskFromId(openvr::vr::k_EButton_System)) != 0),
+				((pSecondaryTrackedRemoteNew->ulButtonPressed & ButtonMaskFromId(openvr::vr::k_EButton_System)) != 0),
+				1, KEY_LCTRL);
+
 			//if in cinema mode, then the dominant joystick is used differently
 			if (!VR_UseScreenLayer() && axisJoystick != -1) 
 			{