New compositor node "Sun Beams"

This allows adding a "fake" sun beam effect, simulating crepuscular rays
from light being scattered in a medium like the atmosphere or deep water.
Such effects can be created also by renderers using volumetric lighting,
but the compositor feature is a lot cheaper and is independent from 3D
rendering. This makes it ideally suited for motion graphics.

The implementation uses am optimized accumulation method for gathering
color values along a line segment. The inner buffer loop uses fixed
offset increments to avoid unnecessary multiplications and avoids
variables by using compile-time specialization (see inline comments
for further details).

release/scripts/startup/nodeitems_builtins.py

@@ -318,6 +318,7 @@ compositor_node_categories = [
         NodeItem("CompositorNodeInpaint"),
         NodeItem("CompositorNodeDBlur"),
         NodeItem("CompositorNodePixelate"),
+        NodeItem("CompositorNodeSunBeams"),
         ]),
     CompositorNodeCategory("CMP_OP_VECTOR", "Vector", items=[
         NodeItem("CompositorNodeNormal"),