This patch changes the default alignment of OverflowBox to center rather than
having a bias towards the upper-left corner.

Fixes #1236

Many of the widgets that use CustomPaint were spamming repaints because
CustomPaint repaints when the identity of the onPaint callback changes, which
it does every build for StatelessComponents.

This patch changes CustomPaint to use a CustomPainter, similar to the new
custom layout widgets. The CustomPainter has a `shouldRepaint` function along
with its `paint` function. This function gives clients explicit control over
when the custom paint object repaints.

I accidentally lost this when extracting RenderOverflowBox from
RenderProxyBox.