object: Get rid of void *ptr field.

Instead, allow dmnsn_object to be embedded in a larger struct.

This gives a consistent 1% speed boost.
Coding style fixes.
camera: Kill the ->ptr field.
light: Get rid of ->ptr field.
pigment: Kill ->ptr field.
pattern: Kill ->ptr field.
interior: Kill ->ptr field.
finish: Remove ->ptr fields.
pool: Add memory pool API.
pool: Rename pool_alloc to palloc.
python: Add simple global pool interface.
scene: Use pool.
pool: Separate dmnsn_palloc and dmnsn_palloc_tidy() APIs.
canvas: Use pool.
camera: Use pool.
light: Use pool.
interior: Use pool.
pattern: Use pool.
map: Use pool.
scene: Get rid of dmnsn_delete_scene().