Add mutex to protect Voodoo swap count and pending. Should fix hangs

sometimes changing video mode on Voodoo 3.

src/vid_voodoo_common.h

@@ -229,6 +229,7 @@ typedef struct voodoo_t
 
         uint32_t tmuConfig;
 
+        mutex_t *swap_mutex;
         int swap_count;
 
         int disp_buffer, draw_buffer;