Add the ability for a shader to write to a texture

It would be very useful and enable awesome effects if shaders could write to textures. See this question from stack exchange - blender.stackexchange.com/questions/212606/is-it-possible-to-read-and-write-to-a-texture-using-osl

An example of what can be done can with other shaders is here - github.com/piellardj/game-of-life-webgl