Lines missing X or Y don't render correctly in single-step preview.

[blurfl]

Presently, if X or Y are absent we use self.gcodecanvas.positionIndicator.pos[] values, but these are over-written by the real-timeposition of the router. This causes the cursor to jump off the cutline on the screen when stepping through the line missing the X or Y value.
If a gcode line does not contain either an X or a Y, we need the previous value to correctly render the cursor during the single-step preview.