initial checkin of Mixes/Strobe.fx shader for lightworks

* allows you to cut between two clips at repeated intervals
* works by using the "_Progress" variable exposed to the shader by lightworks, which gives us a float between 0.0 and 1.0 representing the progress of the effect
* to create a repeating effect, we have to do some lazy math on the cuts, if we had access to any of the frame counts we could use much more accurate math
* because of the above, any changes to the effect length with cause all the strobe timings to change
updating readme with information about Mixes/Strobe.fx