This makes it more consistent with tightFor(), and also makes it
easier to tighten both directions at once when you're not sure you
will always do so (e.g. if you have a height and width that might be
null, and want to tighten whichever ones aren't null).

This patch make grid layout much more flexible. The behavior is factored
out into a GridDelegate that's modeled after the custom layout
delegates. The patch includes a MaxTileWidthGridDelegate that implements
the old behavior and a FixedColumnCountGridDelegate that implements a
grid layout with a fixed number of columns.

Fixes #1048

Previously we used Position, which makes it harder to accumulate offsets
when walking the render tree.

When assigning a new overlayPainter, we were detaching the old overlay
painter even if the render object itself wasn't attached. Now we only
twiddle the attach/detach state of the overlay painter when we're
attached ourselves.

Fixes #1047

Use the same technique for updating compositing bits as layout and
painting. This avoids walking the entire rendering tree when all you
need to update is a small subtree.