Cockpit: backtick (\`) toggles the secondary displays

Add a hide/show toggle for the single-window cockpit's secondary displays --
the five MFDs + radar panel (and their button banks) that frame the 3D
viewscreen -- so the player can get an unobstructed full-window view.

- L4VB16.cpp: gBTHideSecondaryDisplays gates BTDrawCockpitPanel (early return
  skips the whole instrument composite; the primary 3D view already fills the
  window, so nothing else changes).
- btl4main.cpp WndProc: '`' (VK_OEM_3) flips it, edge-triggered on lParam bit 30
  so auto-repeat while held doesn't flicker. Handled in WM_KEYDOWN, which the
  engine keyboard reader does not steal (it consumes WM_KEYUP/WM_CHAR).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

engine/MUNGA_L4/L4VB16.cpp

@@ -650,10 +650,18 @@ int BTCockpitButtonAt(int px, int py, int clientW, int clientH)
 	return -1;
 }
 
+// Backtick (`) toggle: hide the SECONDARY cockpit displays -- the MFD/radar
+// panel (and its button banks) that frame the 3D viewscreen -- for an
+// unobstructed view.  Flipped from the main WndProc (btl4main.cpp) on the '`'
+// key; read here so the composite simply skips itself when hidden.  The primary
+// 3D viewscreen already fills the window, so hiding the panel needs nothing else.
+int gBTHideSecondaryDisplays = 0;
+
 //
 void BTDrawCockpitPanel(LPDIRECT3DDEVICE9 device)
 {
 	if (!DevCockpitPanel() || device == NULL) return;
+	if (gBTHideSecondaryDisplays) return;		// '`' hid the instrument panel
 	IDirect3DSurface9 *rt = NULL;
 	if (FAILED(device->GetRenderTarget(0, &rt)) || rt == NULL) return;
 	D3DSURFACE_DESC d;

game/btl4main.cpp

@@ -80,6 +80,16 @@ LRESULT CALLBACK WndProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
 	switch (uMsg)
 	{
 	case WM_KEYDOWN:
+		// Backtick (`) toggles the SECONDARY cockpit displays (the MFD/radar
+		// panel that frames the 3D viewscreen) on/off, for an unobstructed
+		// view.  Edge-triggered via lParam bit 30 (the previous key state) so
+		// auto-repeat while the key is held does not flicker the panel.
+		if (wParam == VK_OEM_3 && (lParam & (1 << 30)) == 0)
+		{
+			extern int gBTHideSecondaryDisplays;
+			gBTHideSecondaryDisplays = !gBTHideSecondaryDisplays;
+		}
+		return 0;
 	case WM_KEYUP:
 		// Drive keys are NOT read here: the engine's keyboard reader
 		// (L4CTRL.cpp:1506) GetMessage()s every WM_KEYUP / WM_CHAR out of the