Implement the Sketchup::Texture class

[noelwarren]

Details can be found here
https://ruby.sketchup.com/Sketchup/Texture.html

Of particular note:
#average_color ⇒ Sketchup::Color? according to the documentation this can return nil, though I've never seen it do so. I think we can forget the std::optional. Instead return a dummy Sketchup::Color

#image_rep(colorized = false) ⇒ Sketchup::ImageRep implement and return a dummy object for ImageRep

#size=(size) ⇒ Integer, Array(Integer, Integer) Not sure how to best handle the syntax of this in c++. Devs should definitely not have to construct Arrays though (although that will probably be what happens under the hood)