Enhance GUI API

[Xaeroxe]

Currently there is a Canvas struct with a single function that allows for copying whole or partial textures and applying various effects to them. This can technically be used to accomplish just about any of your GUI needs but it could certainly be improved upon. First off all the functions from sdl2's Renderer should be passed through for ease of use. Another realm in which we could improve is by providing layer mask support, which would allow users to display non-square cutouts of textures in their UI.