This fixes PHP bug #73155: https://bugs.php.net/bug.php?id=73155

Patch adapted from upstream source repository:

https://github.com/libgd/libgd/commit/8067a8ac336dfe0acbe96ec2eb24572209a7f279

(.gitignore change removed)

From 8067a8ac336dfe0acbe96ec2eb24572209a7f279 Mon Sep 17 00:00:00 2001
From: "Christoph M. Becker" <cmbecker69@gmx.de>
Date: Fri, 23 Sep 2016 18:29:52 +0200
Subject: [PATCH] Fix #309: gdImageGd2() writes wrong chunk sizes on boundaries

(cherry picked from commit bb1998a16e30d542ab22eba5501911a9aa066edb)
---
 src/gd_gd2.c             |  4 ++--
 tests/gd2/CMakeLists.txt |  1 +
 tests/gd2/Makemodule.am  |  1 +
 tests/gd2/bug00309.c     | 37 +++++++++++++++++++++++++++++++++++++
 4 files changed, 41 insertions(+), 2 deletions(-)
 create mode 100644 tests/gd2/bug00309.c

diff --git a/src/gd_gd2.c b/src/gd_gd2.c
index 75e5e1f..b9b2f93 100644
--- a/src/gd_gd2.c
+++ b/src/gd_gd2.c
@@ -938,8 +938,8 @@ _gdImageGd2 (gdImagePtr im, gdIOCtx * out, int cs, int fmt)
 	};
 
 	/* Work out number of chunks. */
-	ncx = im->sx / cs + 1;
-	ncy = im->sy / cs + 1;
+	ncx = (im->sx + cs - 1) / cs;
+	ncy = (im->sy + cs - 1) / cs;
 
 	/* Write the standard header. */
 	_gd2PutHeader (im, out, cs, fmt, ncx, ncy);
diff --git a/tests/gd2/CMakeLists.txt b/tests/gd2/CMakeLists.txt
index 3b650ad..247b466 100644
--- a/tests/gd2/CMakeLists.txt
+++ b/tests/gd2/CMakeLists.txt
@@ -1,5 +1,6 @@
 SET(TESTS_FILES
 	bug_289
+	bug00309
 	gd2_empty_file
 	gd2_im2im
 	gd2_null
diff --git a/tests/gd2/Makemodule.am b/tests/gd2/Makemodule.am
index b8ee946..d69aee0 100644
--- a/tests/gd2/Makemodule.am
+++ b/tests/gd2/Makemodule.am
@@ -1,5 +1,6 @@
 libgd_test_programs += \
 	gd2/bug_289 \
+	gd2/bug00309 \
 	gd2/gd2_empty_file \
 	gd2/php_bug_72339 \
 	gd2/gd2_read_corrupt
diff --git a/tests/gd2/bug00309.c b/tests/gd2/bug00309.c
new file mode 100644
index 0000000..b649cdc
--- /dev/null
+++ b/tests/gd2/bug00309.c
@@ -0,0 +1,37 @@
+/**
+ * Regression test for <https://github.com/libgd/libgd/issues/309>.
+ *
+ * We test that an image with 64x64 pixels reports only a single chunk in the
+ * GD2 image header when the chunk size is 64.
+ */
+
+
+#include "gd.h"
+#include "gdtest.h"
+
+
+int main()
+{
+    gdImagePtr im;
+    unsigned char *buf;
+    int size, word;
+
+    im = gdImageCreate(64, 64);
+    gdImageColorAllocate(im, 0, 0, 0);
+
+    buf = gdImageGd2Ptr(im, 64, 1, &size);
+
+    gdImageDestroy(im);
+
+    word = buf[10] << 8 | buf[11];
+    gdTestAssertMsg(word == 64, "chunk size is %d, but expected 64\n", word);
+    word = buf[14] << 8 | buf[15];
+    gdTestAssertMsg(word == 1, "x chunk count is %d, but expected 1\n", word);
+    word = buf[16] << 8 | buf[17];
+    gdTestAssertMsg(word == 1, "y chunk count is %d, but expected 1\n", word);
+    gdTestAssertMsg(size == 5145, "file size is %d, but expected 5145\n", size);
+
+    gdFree(buf);
+
+    return gdNumFailures();
+}