Major re-write to drawing routines

Major re-write to drawing routines

FIXME: FULLY OF NASTY API HACKS TO draw.c

Notes from squashed patches:
  Pin and via hole cores are drawn in the appropriate colour
  Playing with stack rendering order
  Fix mask display in 3D (Allow both side masks to display at once)
  Fix rendering depths for silk and mask layers
  Fixup depth for layers and element marks
  Move more rendering to world coordinates
  Fix get_coords function
  Re-write parts of the PCB+GL set_layer function
  Fix layer on/off rendering after commit 9768e060fad7bc3dfc366da76ea1db8154005018
  Only assign stencil planes where sub-compositing is required - that is, not the far-side, mask, rat or switched off layers.
  Pass an explicit graphics context to DrawAttached and DrawMark