[photos][mobile] Performance improvement (#1080)

## Description

Stack has `clipBehaviour = Clip.hardEdge` by default. This is necessary
if content inside the stack is overflowing it's boundary and it has to
be clipped. Clipping is expensive so it's worth removing it when it
makes sense.

In this case of `GalleryFileWidget`, content doesn't overflow the
Stack's boundary so the clip operation can be removed by setting
`clipBehaviour = Clip.none`.

mobile/lib/ui/viewer/gallery/component/gallery_file_widget.dart

@@ -68,6 +68,7 @@ class GalleryFileWidget extends StatelessWidget {
             : _onLongPressNoSelectionLimit(context, file);
       },
       child: Stack(
+        clipBehavior: Clip.none,
         children: [
           ClipRRect(
             borderRadius: BorderRadius.circular(1),