Overlay blend should be performed without a branch

The overlay blend formula is currently

( _dst > 0.5 ? (1.0-(1.0-2.0*(_dst-0.5))*(1.0-_src)) : (2.0*_dst*_src) )

But it can be implemented in the following way with the use of the step function ( http://http.developer.nvidia.com/Cg/step.html ) :

lerp( (2.0*_dst*_src), (1.0-(1.0-2.0*(_dst-0.5))*(1.0-_src)), step(_dst, 0.5) )

This is also discussed in this thread:

http://forum.unity3d.com/threads/overlay-blend-mode-shader.181134/#post-1344111

This is healthier for GPUs, as it trades one branch instruction for two math instructions.

At least, it should be somehow possible to choose between the two.

(Note, I checked the generated GLSL code of the compiled shaderforge, and it indeed results in a branch instruction)