PixelPaint: Add simple "Crop Image to Content" feature

This command finds the smallest non-empty content bounding rect
by looking for the outermost non-transparent pixels in the image,
and then crops the image to that rect.

It's implemented in a pretty naive way, but it's a start. :^)

Userland/Applications/PixelPaint/Layer.cpp

@@ -269,4 +269,50 @@ void Layer::set_edit_mode(Layer::EditMode mode)
     m_edit_mode = mode;
 }
 
+Optional<Gfx::IntRect> Layer::nonempty_content_bounding_rect() const
+{
+    Optional<int> min_content_y;
+    Optional<int> min_content_x;
+    Optional<int> max_content_y;
+    Optional<int> max_content_x;
+
+    for (int y = 0; y < m_content_bitmap->height(); ++y) {
+        for (int x = 0; x < m_content_bitmap->width(); ++x) {
+            auto color = m_content_bitmap->get_pixel(x, y);
+            if (color.alpha() == 0)
+                continue;
+
+            if (!min_content_x.has_value())
+                min_content_x = x;
+            else
+                min_content_x = min(*min_content_x, x);
+
+            if (!min_content_y.has_value())
+                min_content_y = y;
+            else
+                min_content_y = min(*min_content_y, y);
+
+            if (!max_content_x.has_value())
+                max_content_x = x;
+            else
+                max_content_x = max(*max_content_x, x);
+
+            if (!max_content_y.has_value())
+                max_content_y = y;
+            else
+                max_content_y = max(*max_content_y, y);
+        }
+    }
+
+    if (!min_content_x.has_value())
+        return {};
+
+    return Gfx::IntRect {
+        *min_content_x,
+        *min_content_y,
+        *max_content_x - *min_content_x + 1,
+        *max_content_y - *min_content_y + 1
+    };
+}
+
 }