The flame graph view used to draw only so much of the graph that could
be displayed. Change to draw the whole graph, and add a scrollbar.
Does some tricks with the scrolling to keep the bottom of the graph
fixed when resizing or double-clicking, since it works better then.
Layout cleared the list of bars in the flame graph, but didn't clear the
reference m_hovered_bar. This could cause a crash in mousedown_event()
when clicking twice: the first click caused layout, the second used
the old reference.
