Deform a quad image within the boundary of an object using relative vertices within the object.
Note that this is _not_ deformed with perspective (there are no Z values.)
Based on: https://iquilezles.org/www/articles/ibilinear/ibilinear.htm by Inigo Quilez
The four points of the quad deform are the X,Y parameters for the effect. The parameters are relative to the size (width, height) of the original quad / image. 0,0 is the top left. 1,1 is the bottom right.
The deformed quad will not be rendered outside the bound of the original quad. A helper fcn detailed in the examples allows the quad to have vertices outside the original quad size and position.