Add support for a "sample merged" mode

[nididak]

When I work with images I like to keep the original untouched and do my painting/cloning in a dedicated painting layer. "Sample merged" option in tools is very useful for that since, in case of cloning tool, I can just enable it and pick sample region without changing layers since it will just sample from what's visible and paint in the dedicated layer. As it is now, "Heal selection" just errors with "The texture source is empty. Does any selection include non-transparent pixels?".