Fix T51308: Bright/Contrast Doesn't respect Pre-multiplied Alpha

Brightness/contrast node was changing color but did not modify alpha
or ensured colors are premultiplied on the output. This was giving
artifacts later on unless alpha was manually converted.

Compositor is supposed to work in premultiplied alpha (except of
some really corner cases) so it makes sense to ensure premultiplied
alpha after brightness/contrast node.

This is now done as an option enabled by default, so we:

(a) Keep compatibility with old files.
(b) Have correct behavior for newly created files.

Later on we can get rid of this option.

source/blender/editors/space_node/drawnode.c

@@ -2478,6 +2478,11 @@ static void node_composit_buts_sunbeams(uiLayout *layout, bContext *UNUSED(C), P
 	uiItemR(layout, ptr, "ray_length", UI_ITEM_R_SLIDER, NULL, ICON_NONE);
 }
 
+static void node_composit_buts_brightcontrast(uiLayout *layout, bContext *UNUSED(C), PointerRNA *ptr)
+{
+	uiItemR(layout, ptr, "use_premultiply", 0, NULL, ICON_NONE);
+}
+
 /* only once called */
 static void node_composit_set_butfunc(bNodeType *ntype)
 {
@@ -2705,6 +2710,8 @@ static void node_composit_set_butfunc(bNodeType *ntype)
 		case CMP_NODE_SUNBEAMS:
 			ntype->draw_buttons = node_composit_buts_sunbeams;
 			break;
+		case CMP_NODE_BRIGHTCONTRAST:
+			ntype->draw_buttons = node_composit_buts_brightcontrast;
 	}
 }