roundedcorners: Ensure alpha_mem is mutable when updating radius

The `get_mut()` function can return None when the memory is not writable, so
instead, make sure the memory is writable by using `make_mut()`.

Fixes #214

video/videofx/src/border/roundedcorners.rs

@@ -115,7 +115,7 @@ impl RoundedCorners {
         let alpha_stride = out_info.stride()[3];
 
         let mem = &mut state.alpha_mem;
-        let mut_mem = mem.get_mut().unwrap();
+        let mut_mem = mem.make_mut();
 
         let mut alpha_mem = mut_mem
             .map_writable()