Overlay updates: chromium-pdf and thin-manifests

I have slowly been working on cleaning out packages from my overlay, either by dropping them or moving them to the gentoo-x86 tree.

I have gotten a few complaints for dropping the chromium-pdf package which installs the google-chrome pdf viewer for www-client/chromium. I have added that back, and will be bumping the versions within the next day or so. If you also have google-chrome installed, an alternative is to symlink /opt/google/chrome/libpdf.so to /usr/lib/chromium-browser/libpdf.so.

Another change is the conversion to using thin manifests, a feature introduced in portage-2.1.10.16, with some bug fixes in 2.1.10.18 (and the equivalent 2.2_alpha releases). Thin manifests only contain entries for distfiles, and rely on the VCS to validate the integrity of ebuilds. Thanks to Brian and Zac for putting in the work on this feature.

This means you will need a fairly recent version of portage to use my overlay, or you will get errors about missing manifest entries. I have no particular reason for making this change, other than I find the feature to be pretty neat.