New version 1.0.1 in dev
Fix issue #405 - crash after merge down

We were deleting the destination cel if there weren't a source cel.
Fix issue merging frames that weren't rendered on any editor at least once

This bug is because the mask color of cel images were fixed when they
were used in the rendering process. Now, the mask color is fixed when the
image is added to the raster::Stock structure.
Merge branch 'dev'