"global replace" merge mode considered harmful

[mikeroberts3000]

Surprisingly, feedback received from our group of Summer 2013 interns suggests that the "global replace" merge mode is counter-productive. The interns simply did not find this merge mode useful. The interns went so far as to suggest that we should remove the "global replace" merge mode from Mojo to remove the possibility that an inexperienced user would make the mistake of using it.

Since "global replace" is the only merge mode that doesn't touch pixel data (i.e., both other merge modes touch pixel data), this is an issue that warrants rethinking the data structures used to manage merging.

To work around this issue, the interns encouraged each other to never use the merge tool, and instead use the merging functionality that exists in the split tool.