Blender continuous tile saving callback workaround

By default the tile continuous tile saving callback now writes full images after each tile to a tmp file and then moves it to the final destination. 
This is a workaround for Blender's image reading bugs. I haven't seen Blender crash when using Blenderseed after this change on linux.

if APPLESEED_CLI_CONTINUOUSSAVINGTILECALLBACK_TILED is defined and the file format is EXR, 
the callback will write images tile by tile. It works better for applications that can read incomplete tiled EXR files correctly.

PNG files are always written complete independent of the define.
`// Boost headers.`