UI: Change node editor grid into a dot grid

This patch makes the background grid of the node editor a grid of dots
instead of lines. This makes the background look a bit more subtle and
reduces visual complexity. The dots are meant to provide a reference
when panning and zooming. Based on the design of @pablovazquez, and
a patch originally authored by @fabian_schempp.

The "Grid Levels" controls how many levels of dots are drawn. As the
editor zooms in, the higher levels of dots fade in, making them closer
together visually. The zoom factor at which each grid starts and ends
fading in is controllable in the code, and could be tweaked further
in the future. The new default value is 7, out of a range from 0 to 9.

Differential Revision: https://developer.blender.org/D10345

source/blender/editors/include/ED_node.h

@@ -49,7 +49,7 @@ typedef enum {
   NODE_RIGHT = 8,
 } NodeBorder;
 
-#define NODE_GRID_STEPS 5
+#define NODE_GRID_STEP_SIZE 10
 #define NODE_EDGE_PAN_INSIDE_PAD 2
 #define NODE_EDGE_PAN_OUTSIDE_PAD 0 /* Disable clamping for node panning, use whole screen. */
 #define NODE_EDGE_PAN_SPEED_RAMP 1