graphics: custom full screen resolution (#274)

## Link to GitHub Issue, if one exists
n/a

## Description of change
Adds a new option to forcibly set the full screen resolution.

Same idea as -windowscale.

## Compiling
🔺 

## Testing
Seems to be showing similar results as -windowscale:

* Works GREAT for IIDX/SDVX
* popn launches at correct resolution but only draws a small image (at
native res) - can be scaled with F11 menu
* nostalgia is broken in weird ways, image is drawn correctly but bottom
`credits` text draws at wrong offset, game's wintouch logic broken
(perhaps expectedly)

## Other notes
In theory this could enable "play sdvx in landscape mode" option but the
image scaling doesn't quite work with extreme values.

src/spice2x/hooks/graphics/graphics.cpp

@@ -67,6 +67,7 @@ graphics_dx9on12_state GRAPHICS_9_ON_12_STATE = DX9ON12_AUTO;
 bool GRAPHICS_9_ON_12_REQUESTED_BY_GAME = false;
 bool SUBSCREEN_FORCE_REDRAW = false;
 bool D3D9_DEVICE_HOOK_DISABLE = false;
+std::optional<std::pair<uint32_t, uint32_t>> GRAPHICS_FS_CUSTOM_RESOLUTION;
 
 // settings
 std::string GRAPHICS_DEVICEID = "PCI\\VEN_1002&DEV_7146";