Build instructions rely on OF submodule which does not necessarily track HEAD

[artificiel]

The PG build instructions are based on an OF download, but the submodule there is tied to a specific PG commit. Which might make sense in terms of integration on the OF side, but not on the PG dev side.

The instruction in PG repo should produce a HEAD-based build.

Sister issue to untangle too: openframeworks/openFrameworks#7609

[dimitre]

Now 0.12.0 is out I think it is a good time to reavail the need of using projectGenerator (and maybe apothecary) as submodules.
They can be cloned on demand via some script.
This way we always point to head, have simpler PRs for OF without changes in submodules