Changes to Editor::visual_changer to support Item/Canvas::prepare_for_render

Changes to Editor::visual_changer to support Item/Canvas::prepare_for_render

This is necessary to allow calculation of correct intersection of visible
canvas area and items for the new Item::prepare_for_render() API.

samples_per_pixel must be set first to calculate the new horizontal canvas
position in Editor::set_horizontal_position and then
WaveView::set_samples_per_pixel will eventually call
WaveView::prepare_for_render for those items that are visible on the new canvas
position at the new position.

Or if there is not a change to zoom state then call Canvas::prepare_for_render
explicitly.

Also changes so that each method is only called once during
Editor::visual_changer