mtmd : move helpers to dedicated library (⚠️ breaking change)

[ngxson]

First breaking change in libmtmd:

• The mtmd_helpers_* are moved to a dedicated library. If you use the shared library, make sure to link against the new libmtmd_helper_shared with mtmd-helper.h header
• mtmd_helper_bitmap_init_from_* now takes mtmd_context * ctx as the first argument. This is needed to determine the audio bitrate

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Why this change is needed?

• mtmd-helper is not part of the main library. As the name suggested, it provides a higher-level functions that allow developer to get started with libmtmd more easily. Making it a dedicated library to clarify this idea
• miniaudio.h and stb_image.h are 2 large single-header dependencies, which can increase compilation time of the main libmtmd. By moving them to mtmd-helper, we reduce this compilation time during the development (QoL improvement for contributors)

[ggerganov]

Assuming this is mostly code rearrangement. Haven't reviewed in details - let me know if something needs extra attention.

[ngxson]

@ggerganov yes most of the changes are just moving the code around. There could be leftover/redundant things (spotted one on 669fe81), but they won't affect the functionality. I will continue clean things up with my upcoming PRs

[LostRuins]

clip_image_load_from_bytes is removed? Is it not needed anymore?

I still see

llama.cpp/tools/mtmd/clip.h

Line 91 in e562eec

bool clip_image_load_from_bytes(const unsigned char * bytes, size_t bytes_length, struct clip_image_u8 * img);

but where is the implementation?

[ngxson]

A lot of things in clip.cpp/h are redundant and they will be all removed in a future PR