graphics: add option to disable win11 rounded corners on windows (#641)

## Description of change
This adds a toggleable option to remove the rounded window corners when
running a game, or a subscreen, in a window on Windows 11 and upwards.

## Testing
On Windows 11: Resort Anthem, MÚSECA and EPOLIS were tested and the
window corners were removed from both main windows and the TDJ subscreen
when running in windowed mode, with no change in fullscreen mode.

On Windows 7: IIDX18 was tested with no change, the call to
`DwmSetWindowAttribute` is ignored as `DWMWA_WINDOW_CORNER_PREFERENCE`
doesn't exist.

For XP builds, the call is compiled out to avoid linking against dwmapi
and windows_dll_compat_checker returns successfully when testing against
the XP Embedded image. I can't test it on XP, but it ran on Vista.

src/spice2x/cfg/screen_resize.h

@@ -60,6 +60,7 @@ namespace cfg {
         //     window = rectangle including the frame
         //     client = just the content area without frames.
         bool window_always_on_top = false;
+        bool window_disable_round_corners = false;
         bool client_keep_aspect_ratio = true;
         bool enable_window_resize = false;
         int window_decoration = 0; // enum type WindowDecorationMode