Add hb_blob_copy_writable_or_fail()

New API:
- hb_blob_copy_writable_or_fail()

docs/harfbuzz-sections.txt

diff --git a/docs/harfbuzz-sections.txt b/docs/harfbuzz-sections.txt
index a9b4d53..5df33a8 100644
--- a/docs/harfbuzz-sections.txt
+++ b/docs/harfbuzz-sections.txt
@@ -9,6 +9,7 @@
 <FILE>hb-blob</FILE>
 hb_blob_create
 hb_blob_create_sub_blob
+hb_blob_copy_writable_or_fail
 hb_blob_destroy
 hb_blob_get_data
 hb_blob_get_data_writable

src/hb-blob.cc

diff --git a/src/hb-blob.cc b/src/hb-blob.cc
index 8046012..b5291f6 100644
--- a/src/hb-blob.cc
+++ b/src/hb-blob.cc
@@ -171,6 +171,31 @@
 }
 
 /**
+ * hb_blob_copy_writable_or_fail:
+ * @blob: A blob.
+ *
+ * Makes a writable copy of @blob.
+ *
+ * Return value: New blob, or nullptr if allocation failed.
+ *
+ * Since: 1.8.0
+ **/
+hb_blob_t *
+hb_blob_copy_writable_or_fail (hb_blob_t *blob)
+{
+  blob = hb_blob_create (blob->data,
+			 blob->length,
+			 HB_MEMORY_MODE_DUPLICATE,
+			 nullptr,
+			 nullptr);
+
+  if (unlikely (blob == hb_blob_get_empty ()))
+    blob = nullptr;
+
+  return blob;
+}
+
+/**
  * hb_blob_get_empty:
  *
  * Returns the singleton empty blob.

src/hb-blob.h

diff --git a/src/hb-blob.h b/src/hb-blob.h
index 53682d3..fd561f7 100644
--- a/src/hb-blob.h
+++ b/src/hb-blob.h
@@ -83,6 +83,9 @@
 			 unsigned int  length);
 
 HB_EXTERN hb_blob_t *
+hb_blob_copy_writable_or_fail (hb_blob_t *blob);
+
+HB_EXTERN hb_blob_t *
 hb_blob_get_empty (void);
 
 HB_EXTERN hb_blob_t *