Avoid stack-heavy Huffman optimization for fixed deflate blocks

[xiaobai050]

Summary

Construct RFC 1951 fixed literal/length and distance Huffman codes directly when starting a static deflate block. The generic optimizer is still used for dynamic blocks.

The fixed codes are predetermined by the format, but tdefl_start_static_block() currently calls tdefl_optimize_huffman_table() twice. In an ESP-IDF build this call path had a 5,584-byte stack frame, even with static_table=true, which made PNG snapshot compression expensive on a small UI task stack. The code tables already live in tdefl_compressor; this change fills them using scalar locals and reverses each code for the deflate bit order.

Validation

• The same source transformation has been used in a local M5GFX adapter for PNG snapshots; fixed deflate streams were checked with lossless pixel round trips and worst-case inputs in that project.
• The fixed-code ranges and bit lengths follow RFC 1951 section 3.2.6. Dynamic Huffman construction is unchanged.
• Source diff check passed. This standalone upstream branch has not been compiled in this preparation environment.

[ainyan03]

Thank you for this. The fixed codes match RFC 1951, and on the host the compressor gives byte-identical output before and after the change (static and dynamic blocks, 168 inputs).

The files under src/lgfx/ are maintained in LovyanGFX and synced into M5GFX from there, so I have taken your commit (with you as the author) into lovyan03/LovyanGFX#943. It will reach M5GFX with the next sync, so I'm closing this one.

One note on the effect: the peak stack of the built-in createPng does not change, because it uses dynamic blocks, which still run tdefl_optimize_huffman_table(). The saving applies to callers that force static blocks, as in your use.