Do not work with empty layers
Merge remote-tracking branch 'origin/master' into empty-layers

* origin/master:
  fix merge_layers docstring
  Add @vovkasm to AUTHORS.txt
  CI fixes
  fix composition of semi-transparent layers

Conflicts:
	tests/test_pixels.py
Merge pull request #26 from vovkasm/empty-layers

Fix crash when process layer with empty sublayers