Update stb_image from v1.33 to v1.48

Should be straight forward, looks like no big modifications are required. Only change (besides from moving to a header file) is that the `stbi_unsupported_format` method seems to be removed (or is custom for libgdx). This is used in gdx2d to check for progressive jpeg, in which case jpgd is used. This check is not really needed as far as I can tell, because jpgd returns null in case loading fails. So with this PR it will simply try to load the file using stb, if that fails then try to load it using jpgd and if that fails return null.

Tested on win x64 and android.

Fixes #2650