Prevent column headers from redrawing on scroll

Vertical scrolling causes the column headers to redraw, which
drastically reduces performance with a large number of rows and/or
complex custom headers. This is a work-around that prevent the columns
from being re-drawn when vertical scrolling is turned on.
Merge pull request #3 from Areson/bug-columnHeaderRedrawOnVerticalScroll

Prevent column headers from redrawing on scroll