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* rm outdated comment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