Blender Git Loki

Cycles: progressive refine option

Just makes progressive refine :)

This means the whole image would be refined gradually using as much
threads as it's set in performance settings. Having enough tiles is
required to have this option working as it's expected.

Technically it's implemented by repeatedly computing next sample for
all the tiles before switching to next sample.

This works around 7-12% slower than regular tile-based rendering, so
use this option only if you really need it.

This commit also fixes progressive update of image when Save Buffers
option is enabled.

And one more thing this commit fixes is handling display buffer with
Save Buffers option enabled. If this option is enabled image buffer
wouldn't have neither byte nor float buffer until image is fully
rendered which could backfire in missing image while rendering in
cases color management cache became full.

This issue solved by allocating byte buffer for image buffer from
tile update callback.

Patch was reviewed by Brecht. He also made some minor edits to
original version to patch. Thanks, man!