mtmd : move helpers to dedicated file

[ngxson]

Ref discussion: #13366 (comment) (cc @mattjcly)

These helper functions are suppose to be the equivalent of common.cpp, so by moving it to a dedicated file, we can prevent accidentally using internal mtmd API in the helper.

2 exceptions of this rule are:

• mtmd_helper_bitmap_init_from_file
• mtmd_helper_bitmap_init_from_buf

The reason is because these APIs use internal struct from clip.cpp. From libmtmd perspective, we never want to expose clip_context to the outside world. However, the problem with these 2 APIs can be resolved by simply move stb_image to mtmh_helper.cpp. But since this is a breaking change to clip, I'll do it later on.