os_gfx -> report mouse move events; still allow passive mouse interaction if window is unfocused, but have recently received mousemove events

src/df/gfx/df_gfx.h

@@ -689,6 +689,7 @@ struct DF_GfxState
   
   // rjf: frame request state
   U64 num_frames_requested;
+  U64 last_time_mousemoved_us;
   
   // rjf: history cache
   DF_StateDeltaHistory *hist;