[tools] blend dtm use gdal w/python2

as it's the default bindings distributed for most platforms.
[ci] use blender ppa 2.7 + python3.4

increase xvfb window to avoid llvmpipe segfault
xvfb auto-servernum avoid Xvfb failed to start

http://manpages.ubuntu.com/manpages/precise/man1/xvfb-run.1.html