renderer: handle ffmpeg returning a zero aspect ratio

We try to handle aspect ratios in metadata (e.g. stretching 1440x1080 videos
out to 1920x1080 when requested), and FFmpeg will usually return a 1/1
aspect ratio even if it can't determine an aspect ratio. However, as mentioned
in the documentation, sometimes it returns a 0/0 aspect ratio when it can't
determine an aspect ratio, which we didn't handle and would lead to the code
allocating a buffer with a 0px height. This commit handles both 1/1 and 0/0
aspect ratios in accordance with the FFmpeg documentation.

Reference: https://www.ffmpeg.org/doxygen/4.0/structAVFrame.html#a62f9c20541a83d37db7072126ff0060d

app/render/backend/opengl/openglworker.cpp

@@ -104,7 +104,7 @@ void OpenGLWorker::FrameToValue(StreamPtr stream, FramePtr frame, NodeValueTable
     }
 
     // Check frame aspect ratio
-    if (frame->sample_aspect_ratio() != 1) {
+    if (frame->sample_aspect_ratio() != 1 && frame->sample_aspect_ratio() != 0) {
       int new_width = frame->width();
       int new_height = frame->height();