better gif transparency handling

we were testing (x == 1), we should have tested (x & 0x1), thanks
diegocsandrim

see https://github.com/jcupitt/libvips/issues/502

ChangeLog

@@ -1,3 +1,6 @@
+19/8/16 started 8.3.4
+- better transparency handling in gifload, thanks diegocsandrim
+
 30/7/16 started 8.3.3
 - fix performance regression in 8.3.2, thanks Lovell
 - yet more robust vips file reading

configure.ac

@@ -2,7 +2,7 @@
 
 # also update the version number in the m4 macros below
 
-AC_INIT([vips], [8.3.3], [vipsip@jiscmail.ac.uk])
+AC_INIT([vips], [8.3.4], [vipsip@jiscmail.ac.uk])
 # required for gobject-introspection
 AC_PREREQ(2.62)
 
@@ -18,7 +18,7 @@ AC_CONFIG_MACRO_DIR([m4])
 # user-visible library versioning
 m4_define([vips_major_version], [8])
 m4_define([vips_minor_version], [3])
-m4_define([vips_micro_version], [3])
+m4_define([vips_micro_version], [4])
 m4_define([vips_version],
           [vips_major_version.vips_minor_version.vips_micro_version])
 
@@ -38,7 +38,7 @@ VIPS_VERSION_STRING=$VIPS_VERSION-`date`
 #   binary interface changes not backwards compatible?: reset age to 0
 
 LIBRARY_CURRENT=47
-LIBRARY_REVISION=0
+LIBRARY_REVISION=1
 LIBRARY_AGE=5
 
 # patched into include/vips/version.h

libvips/foreign/gifload.c

@@ -4,6 +4,11 @@
  * 	- from svgload.c
  * 25/4/16
  * 	- add giflib5 support
+ * 26/7/16
+ * 	- transparency was wrong if there was no EXTENSION_RECORD
+ * 	- write 1, 2, 3, or 4 bands depending on file contents
+ * 19/8/16
+ * 	- better transparency detection, thanks diegocsandrim
  */
 
 /*
@@ -490,9 +495,10 @@ vips_foreign_load_gif_load( VipsForeignLoad *load )
 			if( ext_code == GRAPHICS_EXT_FUNC_CODE &&
 				extension &&
 				extension[0] == 4 && 
-				extension[1] == 1 ) {
+				(extension[1] & 0x1) ) {
 				/* Bytes are 4, 1, delay low, delay high,
-				 * transparency.
+				 * transparency. Bit 1 means transparency
+				 * is being set.
 				 */
 				gif->transparency = extension[4];