fix alpha computation, followup

Change-Id: I7701368e152ce20adbc05802c7c6518183919d1f

external/libebook/0001-lrf-compute-color-interpolation-coeff.-correctly.patch

+From 584c9bcfed7295e5e0a9d5caafb32a5020b74300 Mon Sep 17 00:00:00 2001
+From: David Tardon <dtardon@redhat.com>
+Date: Mon, 12 Jan 2015 17:10:07 +0100
+Subject: [PATCH] lrf: compute color interpolation coeff. correctly
+
+---
+ src/lib/LRFCollector.cpp | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git a/src/lib/LRFCollector.cpp b/src/lib/LRFCollector.cpp
+index 1c02593..3fbee2e 100644
+--- a/src/lib/LRFCollector.cpp
++++ b/src/lib/LRFCollector.cpp
+@@ -50,7 +50,7 @@ const LRFColor combine(const LRFColor &fg, const LRFColor &bg)
+ {
+   assert(0 == bg.a);
+ 
+-  const double a = fg.a / 255.0;
++  const double a = 1 - fg.a / 255.0;
+ 
+   const double r = (1 - a) * bg.r + a * fg.r;
+   const double g = (1 - a) * bg.g + a * fg.g;
+-- 
+2.1.0
+

external/libebook/UnpackedTarball_libebook.mk

@@ -15,6 +15,7 @@ $(eval $(call gb_UnpackedTarball_set_patchlevel,libebook,0))
 
 $(eval $(call gb_UnpackedTarball_add_patches,libebook, \
     external/libebook/ubsan.patch \
+    external/libebook/0001-lrf-compute-color-interpolation-coeff.-correctly.patch.1 \
 ))
 
 # vim: set noet sw=4 ts=4: