hidden: Use new PieceRenderInstructions and svg massage APIs

hidden: Use new PieceRenderInstructions and svg massage APIs

In general terms:
 * When we need a PieceRenderInstructions get it from piece_pri
   (including from gs.occults) rather than from a Lens.
 * Delete the methods from Lens (and the dummy make_pieceid_visible
   function) so we are sure they aren't used.
 * Use the new massage_prep_piecestate function rather than the
   method on Lens.
 * Consequential changes, including providing mutable gpl in one
   placea, and adding/removing _ prefixes for expectedly used/unsued
   variables.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>