Test all renderers before use

Every renderer except SDL had opted-in for testing due to various
quirks discovered over the years, so just do away with this option
and test all renderers.

app/streaming/video/ffmpeg-renderers/vt_metal.mm

@@ -809,15 +809,6 @@ public:
         return false;
     }
 
-    virtual bool needsTestFrame() override
-    {
-        // We used to trust VT to tell us whether decode will work, but
-        // there are cases where it can lie because the hardware technically
-        // can decode the format but VT is unserviceable for some other reason.
-        // Decoding the test frame will tell us for sure whether it will work.
-        return true;
-    }
-
     int getDecoderColorspace() override
     {
         // macOS seems to handle Rec 601 best