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/genhwaccel.cpp

@@ -48,11 +48,6 @@ void GenericHwAccelRenderer::renderFrame(AVFrame*)
     SDL_assert(false);
 }
 
-bool GenericHwAccelRenderer::needsTestFrame()
-{
-    return true;
-}
-
 bool GenericHwAccelRenderer::isDirectRenderingSupported()
 {
     // We only support rendering via read-back