Visual tweaks to the GPGPU exercises.

* Resize FBOs to fit screen ratio/dimensions.
* Simple gradient map for heat/wave exercises.

@mikolalysenko, I think for the sake of appearance it would be better to
have the FBOs match the screen resolution – it might ordinarily be more correct
to be using power-of-two textures and keep them at the same size, but this is
still kept hidden from the user and does make a big difference in terms of
presentation :)

Not 100% sure about using the gradient map on the heat/wave exercises,
what're your thoughts on that?