Fix multiple calls of `FT_Bitmap_Convert'.

The documentation of `FT_Bitmap_Convert' says that multiple calls do
proper reallocation of the target FT_Bitmap object.  However, this
failed for the sequence

  non-empty bitmap
  empty bitmap
  non-empty bitmap

Reason was that `FT_Bitmap_Convert' only reallocated the bitmap
buffer if it became too small; it didn't make the buffer smaller.
For an empty bitmap following a non-empty one, only the buffer
dimension got set to zero, without deallocation.  If the next call
was a non-empty buffer again, an assertion in `ft_mem_qrealloc' was
triggered.

* src/base/ftbitmap.c (FT_Bitmap_Convert): Always reallocate target
buffer to the correct size.

* docs/CHANGES: Document it.

ChangeLog

@@ -1,3 +1,27 @@
+2017-09-05  Werner Lemberg  <wl@gnu.org>
+
+	Fix multiple calls of `FT_Bitmap_Convert'.
+
+	The documentation of `FT_Bitmap_Convert' says that multiple calls do
+	proper reallocation of the target FT_Bitmap object.  However, this
+	failed for the sequence
+
+	  non-empty bitmap
+	  empty bitmap
+	  non-empty bitmap
+
+	Reason was that `FT_Bitmap_Convert' only reallocated the bitmap
+	buffer if it became too small; it didn't make the buffer smaller.
+	For an empty bitmap following a non-empty one, only the buffer
+	dimension got set to zero, without deallocation.  If the next call
+	was a non-empty buffer again, an assertion in `ft_mem_qrealloc' was
+	triggered.
+
+	* src/base/ftbitmap.c (FT_Bitmap_Convert): Always reallocate target
+	buffer to the correct size.
+
+	* docs/CHANGES: Document it.
+
 2017-09-05  Werner Lemberg  <wl@gnu.org>
 
 	[bdf] Fix size and resolution handling.

docs/CHANGES

@@ -54,6 +54,10 @@ CHANGES BETWEEN 2.8 and 2.8.1
       missing.   Previously,   `SIZE'  was  completely   ignored,  and
       FreeType used heuristic values instead.
 
+    - Multiple calls to `FT_Bitmap_Convert' do work now as advertised.
+      Previously,  they failed with an assertion error if there was an
+      empty bitmap between non-empty ones.
+
 
 ======================================================================
 

src/base/ftbitmap.c

@@ -534,8 +534,7 @@
              (FT_ULong)target->rows > FT_ULONG_MAX / (FT_ULong)target_pitch )
           return FT_THROW( Invalid_Argument );
 
-        if ( target->rows * (FT_ULong)target_pitch > old_size              &&
-             FT_QREALLOC( target->buffer,
+        if ( FT_QREALLOC( target->buffer,
                           old_size, target->rows * (FT_UInt)target_pitch ) )
           return error;