Add support for texture conversion from formats support by COLLADA

[lasalvavida]

For example, ktx may be used in a COLLADA asset, but glTF only supports png, gif, and jpg. If possible, we should convert these textures.

The preferred solution to this is a minimalistic one. The converter build architecture should be as simple as possible. I would prefer not to have to build against a huge library like ImageMagick.