display: make zoom unsigned

The fraction is reduced, so the denominator cannot be negative.

As far as maths are concerned, a negative zoom means the image is
rotated 180 degrees. But we already have orientation to deal with that.
So forbid negative numerator too.

include/vlc_vout_display.h

@@ -94,8 +94,8 @@ typedef struct {
      * It will be applied to the whole display if b_display_filled is set, otherwise
      * only on the video source */
     struct {
-        int num;
-        int den;
+        unsigned num;
+        unsigned den;
     } zoom;
 
     vlc_viewpoint_t viewpoint;

src/video_output/video_output.c

@@ -1638,7 +1638,7 @@ static int ThreadReinit(vout_thread_t *vout,
         state.cfg.display.sar.num = 1;
         state.cfg.display.sar.den = 1;
     }
-    if (state.cfg.zoom.num <= 0 || state.cfg.zoom.den <= 0) {
+    if (state.cfg.zoom.num == 0 || state.cfg.zoom.den == 0) {
         state.cfg.zoom.num = 1;
         state.cfg.zoom.den = 1;
     }