This commit makes the dot grid used as background in the node editor more visually stable when zooming in and out. The dot grid now uses a continuously subdividing pattern, where each level of subdivision divides the previous five times, similar to the line grid in the 3D viewport. The maximum for the "Grid Levels" theme setting is changed to 3, since any further subdivisions are too small to be visible. The "Grid Levels" value for the default themes "Blender Dark" and "Blender Light" is therefore changed to 3, as well. Reviewed By: Hans Goudey, Pablo Vazquez Differential Revision: http://developer.blender.org/D13302