software::scaling::context::run: fix srcSliceH argument

Output height was erroneously used as the srcSliceH argument to
sws_scale, causing the output frame to be partially blank when scaling
down or outright erroring when scaling up. See documentation of sws_scale:

https://www.ffmpeg.org/doxygen/4.0/group__libsws.html#gae531c9754c9205d90ad6800015046d74

Fixes #18.

src/software/scaling/context.rs

@@ -154,7 +154,7 @@ impl Context {
                 (*input.as_ptr()).data.as_ptr() as *const *const _,
                 (*input.as_ptr()).linesize.as_ptr() as *const _,
                 0,
-                self.output.height as c_int,
+                self.input.height as c_int,
                 (*output.as_mut_ptr()).data.as_ptr() as *const *mut _,
                 (*output.as_mut_ptr()).linesize.as_ptr() as *mut _,
             );