Provides few cheaper shader alternatives

[mgsx-dev]

Current PBR shader is a bit greedy, the idea is to provide alternatives for lower quality requirements / lowend devices.
This way, users could still use GLTF workflow and features with some options for rendering quality.
Also, i could be nice to document how to let users provide their own shaders.

Options:

• Gouraud: the libgdx classical shader reworked a bit.
• Phong: inspired by xoppa phong shader, adapted as well.
• PBR (default): the actual PBR shader.

About SpecularGlossiness, GLTF has an extension for that but it's not supported by Blender GLTF exporter. So for now it's required to add this kind of textures at runtime (adding material attributes).
Specular texture attribute should be supported in all 3 shader alternatives.