png: fix over-zealous warnings about transforms

libpng 1.6.0/1.6.1 emits warnings about function ordering when there isn't a
problem, which causes Cairo to fail loading images.  Take the patch from
upstream to fix this.

(From OE-Core rev: 50f823557cb5857dfe585a9da88bb8f61ebfba99)

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-multimedia/libpng/libpng/transform.patch

@@ -0,0 +1,46 @@
+Corrected the test on user transform changes on read. It was in the png_set of
+the transform function, but that doesn't matter unless the transform function
+changes the rowbuf size, and that is only valid if transform_info is called.
+
+Upstream-Status: Backport (will be in 1.6.2)
+                 - b44cd59ad9747069b90c6ff0f76558157a61e26a
+                 - a8715cabd11354da9650dff7c0770686a217b550
+Signed-off-by: Ross Burton <ross.burton@intel.com>
+
+
+diff --git a/pngrtran.c b/pngrtran.c
+index 626f3b0..4485e40 100644
+--- a/pngrtran.c
++++ b/pngrtran.c
+@@ -1056,9 +1056,6 @@ png_set_read_user_transform_fn(png_structrp png_ptr, png_user_transform_ptr
+ {
+    png_debug(1, "in png_set_read_user_transform_fn");
+ 
+-   if (!png_rtran_ok(png_ptr, 0))
+-      return;
+-
+ #ifdef PNG_READ_USER_TRANSFORM_SUPPORTED
+    png_ptr->transformations |= PNG_USER_TRANSFORM;
+    png_ptr->read_user_transform_fn = read_user_transform_fn;
+diff --git a/pngtrans.c b/pngtrans.c
+index b532172..f822324 100644
+--- a/pngtrans.c
++++ b/pngtrans.c
+@@ -782,6 +782,17 @@ png_set_user_transform_info(png_structrp png_ptr, png_voidp
+ 
+    if (png_ptr == NULL)
+       return;
++
++#ifdef PNG_READ_USER_TRANSFORM_SUPPORTED
++   if ((png_ptr->mode & PNG_IS_READ_STRUCT) != 0 &&
++      (png_ptr->flags & PNG_FLAG_ROW_INIT) != 0)
++   {
++      png_app_error(png_ptr,
++            "info change after png_start_read_image or png_read_update_info");
++      return;
++   }
++#endif
++
+    png_ptr->user_transform_ptr = user_transform_ptr;
+    png_ptr->user_transform_depth = (png_byte)user_transform_depth;
+    png_ptr->user_transform_channels = (png_byte)user_transform_channels;

meta/recipes-multimedia/libpng/libpng_1.6.1.bb

@@ -10,6 +10,7 @@ LIBV = "16"
 
 SRC_URI = "${SOURCEFORGE_MIRROR}/project/libpng/libpng${LIBV}/${PV}/libpng-${PV}.tar.xz \
            file://0001-configure-lower-automake-requirement.patch \
+           file://transform.patch \
           "
 SRC_URI[md5sum] = "93fc0b0841ce2db0e6756673e22dafc3"
 SRC_URI[sha256sum] = "5ef57f8b9ef591c8504e2a8f78d31779f0c8f2b34b34d01d533360d2483c8946"