In some cases panels without headers were drawn too wide in sidebars
with region overlap.
Instead of always using the maximum width the view allows, headerless
panels now also use the width calculated in
`panel_draw_width_from_max_width_get`. The function already returns the
correct width in all cases and also takes care of insetting the panels,
when their backdrop needs to be drawn, which is necessary for headerless
panels, too, when there is region overlap.
Reviewed By: Hans Goudey
Differential Revision: http://developer.blender.org/D17194
ab9c34e7e7