SDK / exoplayer

Flushing resets all the texture processors within the `FrameProcessor`. This
includes:

- At the back, the FinalMatrixTextureProcessorWrapper, and its MatrixTextureProcessor
- At the front, the ExternalTextureManager
- All the texture processors in between
- All the ChainingGlTextureProcessorListeners in between texture processors
- All the internal states in the aforementioned components

The flush process follows the order, from `GlEffectsFrameProcessor.flush()`

1. Flush the `FrameProcessingTaskExecutor`, so that after it returns, all tasks queued before calling `flush()` completes
2. Post to `FrameProcessingTaskExecutor`, to flush the `FinalMatrixTextureProcessorWrapper`
3. Flushing the `FinalMatrixTextureProcessorWrapper` will propagate flushing through, via the `ChainingGlTextureProcessorListener`

Startblock:
   has LGTM from christosts
   and then
   add reviewer andrewlewis
PiperOrigin-RevId: 506296469