new function "TextureTypeToString" should use the *ai...* namespace

[umlaeute]

Describe the technical debt

All public C-functions exported by libassimp, use a prefix ai.
This is nice, so for the Debian package we even use a version-script to make sure that only proper "public" symbols can be found in the library (for whatever reasons, using -fvisibility=hidden seems to export too many symbols; but anyhow...)

Now, with assimp-5.2.3, there's a new exported symbol TextureTypeToString (which is even used by the assimp cmdline tool), which obviously lacks the ai-prefix.

I figure this is because the function used to be an often-used (and re-implemented) private function, that was turned into a public function for better re-usability.

See #2964 resp #2989

Better solution

Rename the function to aiTextureTypeToString to have a name consistent with other public C-symbols.

Desktop:

• OS: Debian/GNU
• Browser: wtf
• Version 5.2.3

[kimkulling]

This is correct, good catch!