Push2: Mostly fix scale menu text rendering (#8599)

"Mostly" because there are still some issues with horizontal overlap between
entries, but I think that's something that needs to be addressed separately.

Like earlier issues, this was caused by using a screen-derived or default
cairo/pango context.  I also tinkered with some constants to make things look
sensible to me, and hopefully everywhere since it should always look the same
with the context fix.

libs/surfaces/push2/canvas.h

@@ -51,8 +51,6 @@ class Push2Canvas : public ArdourCanvas::Canvas
 	void queue_resize ();
 	bool vblank ();
 
-	Cairo::RefPtr<Cairo::Context> image_context() { return _context; }
-
 	int rows() const { return _rows; }
 	int cols() const { return _cols; }