begin_x[1] is not the second component of the element, but the element
after the one pointed to begin_x. When begin_x points to the last
element, that line overwrites data past the end of the allocation, which,
during my tests, happened to contain the reference count for the matrix.
Hilarity ensues.

This is fixed upstream in mariusmuja/flann@b615f26, but that fix
would break binary compatibility, so I had to make a different one.

Since the bug isn't quite obvious, here's an explanation. In the const
version of any::cast, if policy is a small_any_policy, its get_value
returns its input argument. So r becomes a pointer to obj, and the
return value is a reference to a local variable, which is invalidated
when the function exits.

facerecognizer interface + white + whitespaces remove

libpng 1.5+ recommends a call to png_set_interlace_handling() if you use
png_read_update_info and png_read_image. It will generate a warning
without it.