On FreeBSD 9.3, the i915 kernel driver lacks a feature (hardware context) required by Mesa 9.2+. This forces us to keep Mesa 9.1 just for FreeBSD 9.3.
The goal of this port is to provide an updated i915 driver and allow us to finally drop Mesa 9.1. The port also packages the Radeon driver: it brings no changes beside an updated dependency to drm2_port.ko (also packaged) instead of drm2.ko.