Fix Linux build.

Add missing include files and remove outdated functions from OS-specific code.
Instead of putting a dummy unistd.h on the include path, disable its inclusion the right way.
Fixed wrong translation of 2x2 and 3x3 matrix initializers from a scalar (e.g. float3x3 m = 0.0)
Merge pull request #40 from zeux/linux-fix

Fix Linux build.
tests: add test for SSAO shader from Unity
tests: add clip() test; failing now for non-floats
Fixed wrong translation of clip() with vector arguments
Merge remote-tracking branch 'upstream/master'