fix composition of semi-transparent layers
Merge pull request #24 from vovkasm/group-layers-merge-transparency-bug

fix composition of semi-transparent layers
CI fixes
Add @vovkasm to AUTHORS.txt
fix merge_layers docstring
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