RTMP: Refactor RTMP extended timestamp.

[winlinvip]

Refactor the extended timestamp code for #4356, and follow the below steps:

1. Read the timestamp from the buffer.
2. Read the extended timestamp if the timestamp indicates that an extended timestamp is present.
3. Skip back 4 bytes if following the RTMP v1 2009 version, which specifies that there is no extended timestamp for Type 3 chunks.
4. Generate the chunk timestamp, treating it as an absolute timestamp for Type 0, or as a delta timestamp for Type 1 or 2.

This improvement will be merged to develop branch only.

[Copilot]

Pull Request Overview

This PR refactors the RTMP extended timestamp handling to align with updated RTMP specifications and improve clarity in the timestamp update logic. Key changes include:

• Removing the obsolete branch for updating the timestamp when no extended timestamp is present.
• Updating comments to clearly differentiate between RTMP v1 2009 and RTMP v1 2012 behaviors.
• Adjusting the logic for reading and processing extended timestamps, including conditionally skipping 4 bytes when necessary.

Comments suppressed due to low confidence (1)

trunk/src/protocol/srs_protocol_rtmp_stack.cpp:1125

• The variable 'timestamp' is referenced before it is declared. Consider computing and declaring 'timestamp' prior to its usage in this condition to ensure proper behavior.

if (!is_first_chunk_of_msg && chunk_extended_timestamp > 0 && chunk_extended_timestamp != timestamp) {