appDisplay: Ensure icon grid is always aligned with the pixel grid

appDisplay: Ensure icon grid is always aligned with the pixel grid

When adaptToSize was called with an odd width, the calculated padding
was not an integer and as a result the icon grid was no longer aligned
to the pixel grid. Similarly it was possible for the page size to be a
non-integer value, which was causing items to not be aligned for
pages > 1.

Fixes https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/4015

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1799>