Move render_windows to per_segment_state_t::render_window
Use partial_range for render list
Optimize rendering

- Make Max_linear_depth_objects const
- Skip setup of object rendering if no objects to render
- Defer setting globals until after conditional confirms a segment will
  render
Skip unnecessary segment rendering phases

Make object rendering traverse only the subrange of segments with
objects.

Make alpha rendering traverse only the subrange of segments with alpha
blended textures.
Use array<> for g3ds_tmap::verts
Split seg/obj vars of find_seg_side_face
Pass vcobjptridx_t to digi_ functions
Mark digi variables static
Rework digi dispatch

Build function tables at compile time.  If !SDLMIXER, compile out the
pointer indirections.  If SDLMIXER && PIE, keep function pointers in
read-only memory.