Fix for textures with alpha channel

We would like to request support for textures with alpha channel. Currently, these are not handled correctly.

From a conversation with Juan Ruiz/ESRI:

I did see some issues with rgba in i3slib::i3s::create_texture_set. The current implementation is encoding rgba as jpg, not png. I3slib::i3s::raw_to_jpg doesn’t support 4 channel textures. Support is “to do”. After updating to encode rgba as png, then there’s an issue with i3slib::i3s::raw_to_png. The implementation is also “to do.” This causes create_node to fail.