This simplifies ChainingGlTextureProcessor as it now only connects a
consuming and a producing GlTextureProcessor rather than a previous,
current, and next GlTextureProcessor.
Also use default no-op implementations of the listeners in
SingleFrameGlTextureProcessor and MediaPipeProcessor to avoid
null-checks.
PiperOrigin-RevId: 466301642
(cherry picked from commit a2166a41)
