[babl] extensions: add rgba8_rgb8 conversion that copies 24/32bit chunks

[Øyvind Kolås <ok src gnome org>]

commit 8a58a48d86cf4daf9edb8de95e857477996deb96
Author: Øyvind Kolås <pippin gimp org>
Date:   Wed Apr 4 14:09:54 2018 +0200

    extensions: add rgba8_rgb8 conversion that copies 24/32bit chunks
    
    Slightly speeding up RGB u8 (without alpha) painting in GIMP, the rgba to rgb
    and reverse conversions are used together with the very fast SIMD conversions
    that operate on chunks of 4 components.

 extensions/gegl-fixups.c |   19 +++++++++++++++++--
 1 files changed, 17 insertions(+), 2 deletions(-)
---
diff --git a/extensions/gegl-fixups.c b/extensions/gegl-fixups.c
index e3bb836..d6ad912 100644
--- a/extensions/gegl-fixups.c
+++ b/extensions/gegl-fixups.c
@@ -447,6 +447,22 @@ conv_rgbAF_lrgba8 (const Babl *conversion,unsigned char *srcc,
     }
 }
 
+static void
+conv_rgba8_rgb8 (const Babl *conversion,unsigned char *src, unsigned char *dst, long samples)
+{
+  long n = samples - 1;
+
+  while (n--)
+    {
+      *(unsigned int *) dst = (*(unsigned int *) src);
+      src   += 3;
+      dst   += 4;
+    }
+  dst[0] = src[0];
+  dst[1] = src[1];
+  dst[2] = src[2];
+}
+
 #define conv_rgb8_rgbAF    conv_rgb8_rgbaF
 #define conv_gamma_rgbaF_gamma_rgbAF   conv_rgbaF_rgbAF
 #define conv_gamma_rgbAF_gamma_rgbaF   conv_rgbAF_rgbaF
@@ -531,10 +547,8 @@ init (void)
 
   o (rgbaF, rgbAF);
   o (rgbAF, rgbaF);
-  
   o (gamma_rgbaF, gamma_rgbAF);
   o (gamma_rgbAF, gamma_rgbaF);
-  
   o (rgbAF, lrgba8);
   o (rgb8,  rgbaF);
   o (rgb8,  rgbAF);
@@ -542,6 +556,7 @@ init (void)
   o (rgbaF, rgb8);
   o (rgbAF, rgb8);
   o (bgrA8, rgba8);
+  o (rgba8, rgb8);
 
   return 0;
 }