various: allow setting footage aspect ratios

We had support for detecting aspect ratios and respecting them in the
render, but this allows people to not only see the aspect ratio in use,
but also override it with their own.

app/codec/frame.cpp

@@ -113,7 +113,11 @@ const rational &Frame::sample_aspect_ratio() const
 
 void Frame::set_sample_aspect_ratio(const rational &aspect_ratio)
 {
-  sample_aspect_ratio_ = aspect_ratio;
+  if (aspect_ratio.isNull()) {
+    sample_aspect_ratio_ = 1;
+  } else {
+    sample_aspect_ratio_ = aspect_ratio;
+  }
 }
 
 const rational &Frame::timestamp() const