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.
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