函数 swr_alloc 

pub unsafe extern "C" fn swr_alloc() -> *mut SwrContext

Allocate SwrContext.

If you use this function you will need to set the parameters (manually or with swr_alloc_set_opts2()) before calling swr_init().

@see swr_alloc_set_opts2(), swr_init(), swr_free() @return NULL on error, allocated context otherwise