Build from local source tree

[Christopher Nielsen]

> I want to try to build Gimp from the latest checkin to the gimp-2-10 branch, which includes a backport of the Wayland fixes which seem to have also fixed the marching ants problem.
> 
> Given the Portfile and files, how can I specify build from my local source tree.

Sorry, I misunderstood your original goal: If you’re interested in a fix for the marching ants problem for GIMP 2.10.28, then this will be much simpler. (For some reason, I was thinking you were trying to update the GIMP 3 prerelease.)

So for this case, we can indeed vet this via `gimp2-devel`, per Bill Cole’s suggestion.