A simple heat haze shader

This is a simle heat haze shader. I don't know how "finished" our effects should be before being added to master, or if it should be added to master but not the layers file or what. Let me know how we are doing this if it is not ready enough. It can be used as is, to try it out together with other demo effects.

This is more of a draft, and need to have a few parameters added before it is really ready. It should at least have parameters for fading in and out, a parameter to controll the effects strength and a parameter to tell the shader where the horizont is.