As per summary: the EMERALD_LIBS and EMERALD_CFLAGS variables substituted in the .pc files are the expansion of what is found by Require:
The problem is that by doing this, -lpng14 is written to emeraldengine.pc and nothing will build against it until rebuilt, when updating to libpng 1.5 (and so on).
I hope I got it right.
+ 14 Sep 2011; Samuli Suominen <email@example.com> emerald-0.8.4-r2.ebuild,
+ Use Requires.private and Libs.private instead of Requires and Libs wrt
+ #380197 by Diego Elio Pettenò
Without revbump, I don't really expect emerald to cause any real problems, with only 1-2 reverse deps (likely not using the engine at all)