Force Voodoo blit

Certain applications using the Voodoo adapter stop blitting
when there's no activity (e.g mouse movement, animation).
This results in a black screen when events like window/full screen
transitions take place. Usually this can be fixed by
moving the mouse or with keyboard inout. This change forces
a blit to refresh the screen.

In addition, added critical sections since they are lighter
than mutexes.

src/include/86box/vid_voodoo_common.h

@@ -489,6 +489,10 @@ typedef struct voodoo_t
         uint64_t time;
         int render_time[4];
 
+        int force_blit_count;
+        int can_blit;
+        lightmutex_t* force_blit_mutex;
+
         int use_recompiler;
         void *codegen_data;