Stabilize auto-resize

The window dimensions are integers, so resizing to fit the content may
not be exact.

When computing the optimal size, it could cause to reduce alternatively
the width and height by few pixels, making the "optimal size" unstable.

To avoid this problem, check if the optimal size is already correct
either by keeping the width or the height.

app/src/screen.c

@@ -113,6 +113,13 @@ get_optimal_size(struct size current_size, struct size content_size) {
         h = MIN(current_size.height, display_size.height);
     }
 
+    if (h == w * content_size.height / content_size.width
+     || w == h * content_size.width / content_size.height) {
+        // The size is already optimal, if we ignore rounding errors due to
+        // integer window dimensions
+        return (struct size) {w, h};
+    }
+
     bool keep_width = content_size.width * h > content_size.height * w;
     if (keep_width) {
         // remove black borders on top and bottom