shiny/widget/node: generalize MarkNeedsPaint to Marks.

An upcoming change will distinguish between 'base' and 'top' paint
passes.

Change-Id: I8e2fcc97c1b7d90302c8f8ad8b3b89ab3bfc0e11
Reviewed-on: https://go-review.googlesource.com/24873
Reviewed-by: David Crawshaw <crawshaw@golang.org>
7 files changed