fix freeze on large dendrograms

This commit fixes a bug where vtkDendrogramItem would hang
under certain circumstances while rendering a large tree.
The problem was in vtkContext2D::ComputeFontSizeForBOundedString.
It was possible for the font size to become negative.  This
resulted in an infinite loop.  An explicit check for a minimum
font size now prevents this from happening.

This commit also contains some minor indentation fixes within
vtkDendrogramItem.cxx.

Change-Id: Idc7b3b010473276df8c31adb43268936f67216f6
Merge topic 'fix_large_tree_performance' into master

2d4f0a02 fix freeze on large dendrograms