Refactor WebGL Post-processing

[kikuchan]

Try to solve the issue: https://github.com/openlayers/openlayers/issues/16120

Summary

• Integrate PostProcessPass into the Helper.
• Refactor PostProcessPass functions to use the Helper functions.
• Manage textureSlot within the Helper and remove the slot argument from bindTexture.
• postProcesses now accepts a callback function as a post-processor.
• Adjust FlowLayer to adapt to the new callback style of postProcesses.

[github-actions[bot]]

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