Provide basic texture creation methods

[shinmili]

All of three major implementations of piston2d-graphics interface, gfx, opengl and glium, have their own empty or from_path functions to create a texture. They are basically doing the same thing: prepare memory and call CreateTexture::create.

I think these functions can be CreateTexture's provided methods, and that will boost piston's backend-agnostic feature.

There are minor differences to be discussed in current backend implementations (such as how they report errors, opengl omitting factory because it is (), or Flips in gfx vs glium, ...), but first I want to check if anyone is interested.