Fix alpha computation (-fsanitize=undefined)

Change-Id: I3f50d3bb85bebdece692e069de73febc2ce63b47

external/libebook/UnpackedTarball_libebook.mk

@@ -11,6 +11,10 @@ $(eval $(call gb_UnpackedTarball_UnpackedTarball,libebook))
 
 $(eval $(call gb_UnpackedTarball_set_tarball,libebook,$(EBOOK_TARBALL)))
 
-$(eval $(call gb_UnpackedTarball_set_patchlevel,libebook,1))
+$(eval $(call gb_UnpackedTarball_set_patchlevel,libebook,0))
+
+$(eval $(call gb_UnpackedTarball_add_patches,libebook, \
+    external/libebook/ubsan.patch \
+))
 
 # vim: set noet sw=4 ts=4:

external/libebook/ubsan.patch

+--- src/lib/LRFCollector.cpp
++++ src/lib/LRFCollector.cpp
+@@ -50,7 +50,7 @@
+ {
+   assert(0 == bg.a);
+ 
+-  const double a = fg.a;
++  const double a = fg.a / 255.0;
+ 
+   const double r = (1 - a) * bg.r + a * fg.r;
+   const double g = (1 - a) * bg.g + a * fg.g;