mtmd : add C public API (#13184)
* init
* wip
* working version
* add mtmd::bitmaps
* add test target
* rm redundant define
* test: mtmd_input_chunks_free
* rm outdated comment
* fix merging issue
* explicitly create mtmd::input_chunks
* mtmd_input_chunk_copy
* add clone()
* add const to various places
* add warning about breaking changes
* helper: use mtmd_image_tokens_get_n_pos
