Assert if cmd.pass not null

[vorg]

Submitting command with null or undefined pass will overwrite parent state and prevents inheriting framebuffer from state which will result in rendering to last used FBO instead of e.g. screen.

const cmdState = this.mergeCommands(parentState, cmd, false)

https://github.com/pex-gl/pex-context/blob/master/index.js#L1093)