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.h

@@ -35,6 +35,7 @@ extern UINT GRAPHICS_FORCE_REFRESH;
 extern std::optional<int> GRAPHICS_FORCE_VSYNC_BUFFER;
 extern bool GRAPHICS_FORCE_SINGLE_ADAPTER;
 extern bool GRAPHICS_PREVENT_SECONDARY_WINDOW;
+extern std::optional<std::pair<uint32_t, uint32_t>> GRAPHICS_FS_CUSTOM_RESOLUTION;
 
 extern graphics_dx9on12_state GRAPHICS_9_ON_12_STATE;
 extern bool GRAPHICS_9_ON_12_REQUESTED_BY_GAME;