Clip drawing in wxRendererGeneric::DrawHeaderButtonContents().

Clip drawing of the icon and bitmap to the header rectangle.

Don't clip the text because we already ellipsize it to avoid overflowing but
do the ellipsization correctly, using wxControl::Ellipsize() instead of the
simple (and wrong) duplicated version here.

Also correct and simplify the alignment calculations.

Closes #12047.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64309 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775