Further fixes to audio/video provider selection

- Fix selection not aborting when the preferred provider returns null
- Fix the default video provider being "ffmpegsource" (lowercase)
  instead of "FFmpegSource", which would trip up provider selection
- More generally, make sure the preferred video provider actually exists
  and fall back to the default (FFmpegSource) if not.

Fixes arch1t3cht#23 .
Fixes arch1t3cht#61 .
Fixes arch1t3cht#83 .

src/audio_provider_factory.cpp

@@ -84,10 +84,12 @@ std::unique_ptr<agi::AudioProvider> SelectAudioProvider(fs::path const& filename
 
 	auto tried_providers = sorted.begin();
 
+	LOG_D("provider") << "Preferred Provider: " << preferred;
 	for (; tried_providers < sorted.end(); tried_providers++) {
 		auto factory = *tried_providers;
 		std::string err;
 		try {
+			LOG_D("provider") << "Trying " << factory->name;
 			auto provider = factory->create(filename, br);
 			if (!provider) {
 				err = "Failed to create provider."; 	// Some generic error message here