ZDoom

As far as I understand the GLDEFS shaders, they're read-only, in that they can't update the source image. I'd like to (try to) make something that continually updates a texture, something like say, Conway's game of life. Is it possible?

If not with shaders, is it possible with ZScript?

This will likely require being able to give fragment shaders write access to a texture, directly.

I would wait until Vulkan support is more mature - Graf has made it clear that he wants to wait to do more generalized shader improvements until then.

Writing to a texture with a shader is certainly possible - but it's not this simple. It will require a similar setup like the camera textures or the postprocessing effects that set up a texture as a render target.
The input textures to a shader are immutable objects and cannot be modified.

This also needs to run outside of the main shader, of course.