[gimp] app: Improve math of unaccelerated OperationNormalMode

[Daniel Sabo <daniels src gnome org>]

commit 39a417d4762924f036b9be8e16d792e90f75b4e8
Author: Daniel Sabo <DanielSabo gmail com>
Date:   Mon Jan 11 01:59:12 2016 -0800

    app: Improve math of unaccelerated OperationNormalMode
    
    Hopefully no one is stuck on a CPU old enough to need this, but if
    they are the ratio based math is noticeably faster.

 app/operations/gimpoperationnormalmode.c |    6 +++---
 1 files changed, 3 insertions(+), 3 deletions(-)
---
diff --git a/app/operations/gimpoperationnormalmode.c b/app/operations/gimpoperationnormalmode.c
index baf3b0b..fa44cc3 100644
--- a/app/operations/gimpoperationnormalmode.c
+++ b/app/operations/gimpoperationnormalmode.c
@@ -202,13 +202,13 @@ gimp_operation_normal_mode_process_pixels_core (gfloat              *in,
 
       if (out[ALPHA])
         {
-          gfloat in_weight       = in[ALPHA] * (1.0f - aux_alpha);
-          gfloat recip_out_alpha = 1.0f / out[ALPHA];
+          gfloat aux_weight = aux_alpha / out[ALPHA];
+          gfloat in_weight  = 1.0f - aux_weight;
           gint   b;
 
           for (b = RED; b < ALPHA; b++)
             {
-              out[b] = (aux[b] * aux_alpha + in[b] * in_weight) * recip_out_alpha;
+              out[b] = aux[b] * aux_weight + in[b] * in_weight;
             }
         }
       else