[glTF] Load embedded textures with data URI scheme

[footballhead]

Using scene::GltfLoader, some models in glTF-Sample-Assets fail to load because the texture is embedded with the data URI scheme.

From the spec 5.18.1:

Instead of referencing an external file, [image.uri] MAY contain a data:-URI

This would allow us to load more models like:

• DamagedHelmet (glTF-Embedded variant)
• Duck (glTF-Embedded variant)
• BoxTextured (glTF-Embedded variant)

[footballhead]

cgltf supports a variety of URI, string, and buffer functions for this purpose