[Fluxus] 2d overlay + commits

[gabor papp]

since i could not find a simple way to do 2d overlays in fluxus i added 
a new primitive. (build-image) takes a texture, the location of the 
image top-left corner in pixels, and the size you want the image to be 
displayed in pixels. since it is intended for backgrounds and overlays, 
depth test is disabled while it is rendered.

suggestions and objections are welcome.

best,
gabor

;;;;;;;

(clear)

(build-image (load-texture "test.png")
              (vector 0 0) (get-screen-size))

(build-cube)