app_server: PictureBoundingBoxPlayer fixups

* TODO: squash commit before merge into master

app_server: add picture player for determining bounding box

* Add PictureBoundingBoxPlayer, a new player for BPictures. Instead
of drawing the picture, it determines an approximate bounding box
of its contained drawing operations.

* To increase performance, the resulting bounding box is an
approximation: it guarantees to always enclose all pixels of the
picture, however not necessarily tightly.

* PictureBoundingBoxPlayer::Play() gets a DrawState which is the
initial state used when playing the picture. The player does not
modify this state (it uses a copy internally), so the method is
idempotent.