popn: fix subscreen redraw option causing graphical glitches (#854)

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

## Description of change
Old behavior: Forced redraw presented the subscreen every main frame,
even when the game already presented it, causing duplicate presents and
tearing in popn (was fine in sdvx)

New behavior: Forced redraw acts as a fallback, presenting only when the
game skips or fails a subscreen update.

## Testing
Popn - no more glitching
Nabla - no regression

src/spice2x/hooks/graphics/backends/d3d9/d3d9_backend.h

@@ -15,6 +15,8 @@ void graphics_d3d9_on_present(
     IDirect3DDevice9 *device,
     IDirect3DDevice9 *wrapped_device);
 
+void graphics_d3d9_notify_subscreen_present();
+
 IDirect3DSurface9 *graphics_d3d9_ldj_get_sub_screen();
 
 struct WrappedIDirect3D9 : IDirect3D9Ex {