Replaces length memory blocks in buffer starting at idx with mem.
If length is -1, all memory starting from idx will be removed and replaced with mem.
buffer should be writable.
an index
a length, should not be 0
a #GstMemory
Replaces length memory blocks in buffer starting at idx with mem.
If length is -1, all memory starting from idx will be removed and replaced with mem.
buffer should be writable.