GlimpseDataPainter2D should not hide GlimpseContext

[ulmangt]

GlimpseDataPainter2D.paintTo() passes a GL handle but not the entire GlimpseContext. This makes it impossible to call the paintTo() method of other delegate painters which are not GlimpseDataPainter2D.

Suggest potentially moving the bounds.isValid() check up to GlimpsePainter2D, removing the axis null checks (which are redundant because GlimpsePainter2D makes those checks), and having users override paintTo() and call super.paintTo().