Add HDR support with software decoding using libplacebo

Not supported on macOS yet.

app/streaming/session.h

@@ -172,10 +172,16 @@ private:
 
     void updateOptimalWindowDisplayMode();
 
+    enum class DecoderAvailability {
+        None,
+        Software,
+        Hardware
+    };
+
     static
-    bool isHardwareDecodeAvailable(SDL_Window* window,
-                                   StreamingPreferences::VideoDecoderSelection vds,
-                                   int videoFormat, int width, int height, int frameRate);
+    DecoderAvailability getDecoderAvailability(SDL_Window* window,
+                                               StreamingPreferences::VideoDecoderSelection vds,
+                                               int videoFormat, int width, int height, int frameRate);
 
     static
     bool chooseDecoder(StreamingPreferences::VideoDecoderSelection vds,