This commit modifies the TileManager to sort render tiles once after tiling the image,
instead of searching the next tile every time a new tile is acquired by a device.
This makes acquiring a tile run in constant time, therefore the render time is linear
w.r.t. the amount of tiles, instead of the quadratic dependency before.
Furthermore, each (logical) device now has its own Tile list, which makes acquiring
a tile for a specific device easier.
Also, some code in the TileManager was deduplicated.
Reviewers: dingto, sergey
Differential Revision: https://developer.blender.org/D1684
548eb9eb4b