gitadora: allow a landscape monitor to display subscreen (#899)

## Link to GitHub Issue or related Pull Request, if one exists
#0

## Description of change
Arena Model's SMALL touch subscreen is a portrait (800x1280) panel -
however, it's common for people to set up a touch screen in landscape
orientation since IIDX/SDVX/popn all use landscape.

Add a new option, `-gdsublandscape`, which forces the subscreen to
launch at 1080p landscape. The image is rendered with black bars on the
sides. Touches will be unaligned initially; user needs to go into test
mode and recalibrate. In reality this option is just a short hand for
`-forceressub 1920,1080`.

Wire up `-forceressub` which previously didn't work for gitadora, so
that a resolution other than 1080p can be used in either orientation. In
other games `-forceressub` just forces the rendering resolution and
IIDX/SDVX/popn's game engine automatically stretches to fill... however
for gitadora it works differently since we want to keep the aspect ratio
of the original screen; it instead hijacks the back buffer and makes the
game draw to an intermediate buffer.

## Testing

src/spice2x/games/gitadora/gitadora.h

@@ -1,6 +1,7 @@
 #pragma once
 
 #include <optional>
+#include <utility>
 
 #include <windows.h>
 #include <mmreg.h>
@@ -21,6 +22,7 @@ namespace games::gitadora {
     extern std::optional<socd::SocdAlgorithm> PICK_ALGO;
     extern std::optional<uint8_t> ARENA_WINDOW_COUNT;
     extern bool ARENA_TWO_HEAD_EXCLUSIVE;
+    extern bool ARENA_SUBSCREEN_LANDSCAPE;
     extern std::optional<std::string> ASIO_DRIVER;
     extern bool ALLOW_REALTEK_AUDIO;
     extern bool NATIVE_TOUCH;
@@ -29,6 +31,18 @@ namespace games::gitadora {
     static constexpr int ARENA_SUBSCREEN_WIDTH = 800;
     static constexpr int ARENA_SUBSCREEN_HEIGHT = 1280;
 
+    // used when a landscape monitor drives the SMALL head and -forceressub is unset
+    static constexpr int ARENA_SUBSCREEN_LANDSCAPE_WIDTH = 1920;
+    static constexpr int ARENA_SUBSCREEN_LANDSCAPE_HEIGHT = 1080;
+
+    // resolution the SMALL head actually runs at; the portrait panel size unless
+    // -forceressub or the landscape option overrides it
+    std::pair<UINT, UINT> arena_subscreen_host_size();
+
+    // area of the host the portrait subscreen occupies, centered and aspect-preserved;
+    // whatever is left over on either side stays black
+    RECT arena_subscreen_content_rect(LONG host_width, LONG host_height);
+
     class GitaDoraGame : public games::Game {
     public:
         GitaDoraGame();