graphics: submonitor refresh rate option (#507)

## Description of change
Add an option that forces requested refresh rate value for the second
monitor; i.e., instead of 60Hz allow the user to override the adapter to
be 70Hz.

Useful for people who have a touch screen that can't do 60Hz.

I don't know if this would cause any timing issues in IIDX or SDVX, but
it seems to boot and monitor check is unaffected.

## Testing
Tested TDJ And UFC mode with a submonitor that can only do 61Hz.

src/spice2x/hooks/graphics/graphics.cpp

@@ -63,6 +63,7 @@ bool GRAPHICS_WINDOWED = false;
 std::vector<HWND> GRAPHICS_WINDOWS;
 std::optional<HWND> GRAPHICS_WINDOW_MAIN;
 UINT GRAPHICS_FORCE_REFRESH = 0;
+std::optional<uint32_t> GRAPHICS_FORCE_REFRESH_SUB;
 std::optional<int> GRAPHICS_FORCE_VSYNC_BUFFER;
 bool GRAPHICS_FORCE_SINGLE_ADAPTER = false;
 bool GRAPHICS_PREVENT_SECONDARY_WINDOW = false;