Previously, the canvas size would get set to too small because
 - in the FigureCanvasQt.__init__ `get_width_height` is used to set
   the initial size of the canvas
 - `get_width_height` accounts for the dpiratio, but it has not been
   multiplied up yet so it reports half the size it should
 - this get propagated back down into the Figure object in the resize
   method
 - the dpi is then scaled up

This fix
 - moves adding the `_original_dpi` attribute to the Figure as
   soon as possible in the base __init__
 - calls `_update_dpi` before asking the figure how big it is

Closes matplotlib#8717