Point type

inspired by
https://github.com/mozilla-servo/rust-geom/blob/master/point.rs
implement point scaling
implement point subtraction
use our own Point instead of geom::point::Point2D
Proj.project works with Point objects
implement casting to different Point type
use Point arguments for read_raster
use Point arguments for write_raster
store buffer size as Point
make sure the buffer size is sane
basic worker queue implementation
send MessageToWorker instead of raw integer
rename variables, move imports to the top
rewrite dispatcher loop
dispatcher has its own set of messages
dispatcher knows how many tasks to halt
start dispatcher before workers
register worker with dispatcher
create workers on demand
separate worker channel creation