Improve get2DViewdirection

[rgrieder]

The function mentioned can be found in util/Math.cc line 123. It uses only Vector3 as input/output, but makes heavy use of sin, cos and acos functions. This is quite expensive considering performance (tested with a profiler).
What can you do? According to my experience with such problems, you can always replace the of the triangular functions with standard operations and square roots given that neither input nor output contains any angle values (which is the case here).

[scheusso]

replaced acos, cos and sin through sqrt and *

if you like my solution you are welcome to close this ticket