Blender Git Loki

Cycles: Fix Tile access in the TileManager for viewport rendering

- When rendering in the Viewport, next_tile is sometimes called after a reset has been performed, but before
new tiles were generated. In that case, the tile list would be invalid, causing Blender to crash randomly.
- When generating new tiles, the TileManager would not clear the tile lists before re-generating them, leading
to some tiles being skipped during viewport rendering.
- When popping the next tile from a tile list, a reference to the just-deleted object would be returned, now the
object is copied before deleting it.