Function fz_resize_buffer 

pub unsafe extern "C" fn fz_resize_buffer(
    ctx: *mut fz_context,
    buf: *mut fz_buffer,
    capacity: usize,
)

Ensure that a buffer has a given capacity,
truncating data if required.

capacity: The desired capacity for the buffer. If the current
size of the buffer contents is smaller than capacity, it is
truncated.