Compile with Ubuntu 13.10's incompatible png.h.
Calling update twice severly reduces performance on my system.

It locks the CPU usage at aroung 60% on my dualcore system. With
calling update just once it makes use of 80% of the CPUs and runs much
smoother.
Quad holding physics texture does not show without glFlush.

On my Mesa GL setup (default of Ubuntu 13.10 with AMD HD6670).

OpenGL vendor string: X.Org
OpenGL renderer string: Gallium 0.4 on AMD TURKS
OpenGL core profile version string: 3.1 (Core Profile) Mesa 9.2.1
OpenGL core profile shading language version string: 1.40
Merge branch 'simple-simulation-with-fixes' of https://github.com/gregor-b/ManiacLab into gregor-b-simple-simulation-with-fixes