improved backend encapsulation

Various backend improvements are included in this commit, mostly for the
benefit of exporting. These include:
- Moving more non-GL code from OpenGL derivatives into base classes
- An "export mode" that changes the cache behavior of video backends
- Using the Viewer's UUID introduced a few commits ago
- No longer hardcoding the pixel format/render mode in the backend (since
  they'll inevitably differ when exporting vs previewing)
- Improved signalling for frames that are completed

app/render/backend/audiorenderbackend.cpp

@@ -55,9 +55,6 @@ void AudioRenderBackend::ConnectViewer(ViewerOutput *node)
 {
   connect(node, SIGNAL(AudioChangedBetween(const rational&, const rational&)), this, SLOT(InvalidateCache(const rational&, const rational&)));
   connect(node, SIGNAL(AudioGraphChanged()), this, SLOT(QueueRecompile()));
-
-  // FIXME: Hardcoded format, though I doubt this will change any time soon (maybe we'll shift to DBL at some point)
-  SetParameters(AudioRenderingParams(node->audio_params(), SAMPLE_FMT_FLT));
 }
 
 void AudioRenderBackend::DisconnectViewer(ViewerOutput *node)
@@ -117,3 +114,8 @@ QString AudioRenderBackend::CachePathName()
 
   return this_cache_dir.filePath(QStringLiteral("pcm"));
 }
+
+bool AudioRenderBackend::CanRender()
+{
+  return params_.is_valid();
+}