fix debug engine incorrectly using visual run state to soft halt, when ctrl thread did not necessarily report it was running; was leading to 'phantom halts'; other small fixes, dead code elimination, and begin sketching out proper cross-window drag/drop

src/os/gfx/linux/os_gfx_linux.c

@@ -448,6 +448,12 @@ os_get_modifiers(void)
   return 0;
 }
 
+internal B32
+os_key_is_down(OS_Key key)
+{
+  return 0;
+}
+
 internal Vec2F32
 os_mouse_from_window(OS_Handle handle)
 {