Add a libplacebo-based Vulkan renderer for HDR and Vulkan video decoding support

Vulkan H.264/HEVC video decoding isn't supported with Fedora's Mesa binaries due to
patent issues, so only the VAAPI/DRM-PRIME import path was tested locally with ANV.

HDR video is getting tonemapped to my SDR monitor reasonably, so HDR output probably
works with GameScope but I haven't confirmed yet.

See #1117

app/streaming/video/ffmpeg-renderers/renderer.h

@@ -233,6 +233,15 @@ public:
         return true;
     }
 
+    // Allow renderers to expose their type
+    enum class RendererType {
+        Unknown,
+        Vulkan
+    };
+    virtual RendererType getRendererType() {
+        return RendererType::Unknown;
+    }
+
     // IOverlayRenderer
     virtual void notifyOverlayUpdated(Overlay::OverlayType) override {
         // Nothing