This is an implementation of Enhanced Subpixel Morphological Antialiasing (SMAA)

The algorithm was proposed by:
  Jorge Jimenez, Jose I. Echevarria, Tiago Sousa, Diego Gutierrez

This node provides only SMAA 1x mode, so the operation will be done with no spatial
multisampling nor temporal supersampling. See Patch for comparisons.

The existing AA operation seems to be used only for binary images by some other nodes.
Using SMAA for binary images needs no important parameter such as "threshold", so we
perhaps can switch the operation to SMAA, though that changes existing behavior.

Notes:
1. The program code assumes the screen coordinates are DirectX style that the
   vertical direction is upside-down, so "top" and "bottom" actually represent bottom
   and top, respectively.

Thanks for Habib Gahbiche (zazizizou) to polish and finalize this patch.

Reviewed By: jbakker

Differential Revision: https://developer.blender.org/D2411