stb_image_write: fix linker errors in HDR image functions when STBI_WRITE_NO_STDIO is defined

[sanjay-nambiar]

Calling any of the HDR image write functions with STBI_WRITE_NO_STDIO defined results in an undefined symbol linker error. This is because all the HDR write function definitions are guarded with STBI_WRITE_NO_STDIO macro when only the function that writes directly to a file - stbi_write_hdr should be under the include guard.

Fixing this requires removing one usage of sprintf used when writing an HDR image header. Implemented a simple unsigned int to string function to replace sprintf, borrowing code from stb_sprintf.

[nothings]

Do not borrow from stb_sprintf; that function is not designed to be simple, but to be fast, and we don't need speed here. A much shorter function will do.

[sanjay-nambiar]

Updated the function to be a much simpler one. Thanks!