Make area_t smaller so that it fits in an uint64_t using 4 {u,}int16_t's
for it. Note that xcb uses the very same structure, so we aren't loosing
any information while remaining very small.
This is better to use arrays in term of:
* memory access when iterating over area_t's;
* allocation because area_t's have no *next/*prev members anymore, which
makes it a tiny structure (8 octets);
* allocation because we allocate area_t's by vector of'em rather than one
by one.
Signed-off-by: Pierre Habouzit <madcoder@debian.org>
