Fix axes bounding box not obeying 'layer' property. * gl-render.cc (draw_axes_boxes): Check layer2Dtop to decide which Z-value to place axis at. In 2-D plot, don't draw box lines at second Z-value which overlaps original axis and is always on top. * graphics.cc (update_axes_layout): Set property is2D based on whether view() elevation is 90 degrees. * graphics.in.h (axes::properties): New is2D bool property. * graphics.in.h (axes::properties::get_is2D): New getter function.

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