iconGrid: Eliminate JavaScript for painting/picking

iconGrid: Eliminate JavaScript for painting/picking

The only reason for `vfunc_paint` and `vfunc_pick` existing was to
implement a culling optimization. Although that optimization actually
made performance worse than none at all because it forced the painting
and picking cycles to spend more time calling into JavaScript.

Turns out we don't have to choose between native code and culling though.
Just reimplement the culling using native ClutterActor functions and we
get the benefits of both.

Performance on an i7-7700:

Moving the cursor over the icon grid:
Before: 70% CPU, 5.5ms per frame
After : 60% CPU, 4.5ms per frame

Scrolling the icon grid:
Before: 60% CPU, 4.4ms per frame
After : 50% CPU, 3.3ms per frame

Helps with https://gitlab.gnome.org/GNOME/gnome-shell/issues/174