[gimp] app: orient the spiral op line controller so that 0-degree rotation points right

[N/A <ell src gnome org>]

commit 9de19a1f9a81586e59f4401199358fc5782e436d
Author: Ell <ell_se yahoo com>
Date:   Sun Jul 2 19:10:56 2017 -0400

    app: orient the spiral op line controller so that 0-degree rotation points right
    
    This way, it runs to the middle of the spiral arm, and stays centered
    even when balance != 0.

 app/widgets/gimppropgui-spiral.c |    6 +++---
 1 files changed, 3 insertions(+), 3 deletions(-)
---
diff --git a/app/widgets/gimppropgui-spiral.c b/app/widgets/gimppropgui-spiral.c
index a0d652c..307a638 100644
--- a/app/widgets/gimppropgui-spiral.c
+++ b/app/widgets/gimppropgui-spiral.c
@@ -56,7 +56,7 @@ line_callback (GObject       *config,
   x        = x1 / area->width;
   y        = y1 / area->height;
   radius   = sqrt (SQR (x2 - x1) + SQR (y2 - y1));
-  rotation = atan2 (x2 - x1, y2 - y1) * 180 / G_PI;
+  rotation = atan2 (-(y2 - y1), x2 - x1) * 180 / G_PI;
 
   if (rotation < 0)
     rotation += 360.0;
@@ -93,8 +93,8 @@ config_notify (GObject          *config,
 
   x1 = x * area->width;
   y1 = y * area->height;
-  x2 = x1 + sin (rotation * (G_PI / 180.0)) * radius;
-  y2 = y1 + cos (rotation * (G_PI / 180.0)) * radius;
+  x2 = x1 + cos (rotation * (G_PI / 180.0)) * radius;
+  y2 = y1 - sin (rotation * (G_PI / 180.0)) * radius;
 
   set_func (set_data, area, x1, y1, x2, y2);
 }