Save images to PNG files

[Alexeyt89]

Issue description
I'm implementing glTF exporter with using tinygltf. I need to make tinygltf write each image to separate png files.
On exporting models png files are not created.

To Reproduce
I include tinygltf this way:

#define TINYGLTF_IMPLEMENTATION
#define STB_IMAGE_IMPLEMENTATION
#define STB_IMAGE_WRITE_IMPLEMENTATION
#include "tiny_gltf.h"

Then write all needed data to tinygltf::Image:

mimeType = "image/png"
name = "test"
uri = "test.png"
width = 512
height = 512
write the image data to image.image
bits = 8
component = 4
pixel_type = TINYGLTF_COMPONENT_TYPE_UNSIGNED_SHORT

To save models I call WriteGhtfSceneToFile with these parameters:

embedeImages = false;
embedeBuffers = false;
prettyPrint = true;
writeBinary = false;

As a result I see 2 files (png files are not created):

test.gltf
test.bin

On exporting I have this call stack:

1. tinygltf::TinyGLTF::WriteGhtfSceneToFile
2. tinygltf::UpdateImageObject
3. tinygltyf::WriteImageData <- here it fails to write png files because fsPtr is nullptr

fsPtr is nullptr because this->write_image_user_data_ is nullptr.
Is there a description or an example how png files can be created?

[syoyo]

Writing images to a file(i.e. do not embed images) is not well documented and supported.
Currently you'll need to set the pointer to FsCallback as a user_data through SetImageWriter()

There is a room for improving implementation(Use FsCallback from SetFsCallbacks) and documentation.
Contribution is welcome!

[Alexeyt89]

If somebody needs it, I set it this way:

tinygltf::TinyGLTF gltf_ctx;
tinygltf::WriteImageDataFunction WriteImageData = &tinygltf::WriteImageData;
tinygltf::FsCallbacks fs = {
&tinygltf::FileExists, &tinygltf::ExpandFilePath,
&tinygltf::ReadWholeFile, &tinygltf::WriteWholeFile,
nullptr
};
gltf_ctx.SetImageWriter(WriteImageData, &fs);

I tested it with PNG, JPG and BMP. The only visible problem is in bmp semi-transparent textures are purple.