New port: graphics/libxatracker

XA Tracker is built from Mesa sources and is required by xf86-video-vmware
in order to take advantage of accelerated graphics using the vmwgfx kernel
DRM driver.

This commit also adds libdrm as a runtime requirement for xf86-video-vmware
and patches the DRI device loading routine to work outside of Linux. If no
DRI device exists (vmwgfx kernel driver is not loaded), xf86-video-vmware
will fallback to software rendering.

The vmwgfx kernel driver is available in the drm-kmod port.

Reviewed by: zeising
Approved by: imp (mentor), zeising
Differential Revision: D20153