Simplify drawing when showing of materials has been turned off

We can simplify the drawing code in wings_draw if we let
wings_draw_setup test for materials turned off.
wings_draw_setup: Refactor pointer API

As a preparation for supporting Vertex Buffer Objects in a future
commit, we want to hide more details of #vab{} record. The first
step is replace all the functions that enable or disable pointers
by two functions:

   enable_pointers(#vab{}, [Extra]).
   disable_pointers(#vab{}, [Extra]).

By using one call to enable all pointers, we can be sure that buffers
are set up in the correct order (the VBO must be bound before any
pointers start to reference it). Since the calls are passed the
entire #vab{} record, we don't have to shoehorn the VBO identifier
into #vab.face_vs, but can put it into its own field in the record.
fixup! wings_draw_setup: Refactor pointer API
wings_draw_setup: Introduce get_vertex_pointer/1

To continue the information hiding, introduce get_vertex_pointer/1
to hide the internal representation of the vertex array.
Internally refactor wings_draw_setup

Factor out the creation of the #vab{} record into the create_vab()
function; that will make it easier to introduce VBOs in a future
commit.
Garbage-collect #vab{} records
Teach wings_dl to understand drawing funs

Display lists are deprecated in OpenGL 3.x and later (and not supported
at all in OpenGL 3.x on the Mac). Thus, in the long term we will need to
stop using display lists.

In the short term, displays don't work well in combination with
Vertex Buffer Objects with some OpenGL drivers.

Therefore, we'll teach wings_dl to call drawing funs as a replacement
for display lists.

Note that we need not worry about garbage-collecting those funs.
Even if they happen to contain references to #vab{} records with embedded
references to VBOs, wings_dl already support garbage-collecting #vab{}
records, and wings_draw is careful to always clear display lists at the
same time that #vab{} records are cleaned.
Use Vertex Buffer Objects