Screen: Make video refresh calibration more robust on broken operating systems.

This is another layer of band-aid to keep OSX in the game. The latest
version of OSX 10.9 Mavericks has rather horrible noisy timing on flip
completion, at least on my early 2010 MacBookPro with NVidia GeForce GT
330M as of OSX 10.9.0. This causes many outlier samples during initial
video refresh calibration. As OSX doesn't report a nominal video
refresh rate on internal flat panels, ifi_nominal can't be used to
filter out outlier samples. The result is frequent sync failure due to
outliers - about 80% of all runs!

Now we add the use of ifi_beamestimate as fallback hint. If ifi_nominal
is unavailable, the ifi_beamestimate from beamposition based video
refresh measurement is used as filter instead. This works on my MBP
with its NVidia card, as the PTB kernel driver supports NVidia cards.
It would also work on AMD cards. However, owners of Intel integrated
graphics cards would be out of luck, as neither OSX nor our kernel
driver support beampos queries on Intel gpu's, so no stabilization for
them.

With this commit i only get occassional sync failures, not the insane
amounts as before.

-> Screen for Octave and Matlab on OSX rebuilt and tested. This patch
should also theoretically help on MS-Windows or Linux but is rarely (if
ever) needed there, as those os'es don't have the totally broken
nominal framerate reporting as Apple's masterpiece.