ffmpeg initializers were left in superclass values

which were incorreect for this subclass resulting in
a crash.

IO/FFMPEG/Testing/Data/Baseline/TestFFMPEGVideoSourceWithAudio.png.sha512

-ce01471ff2d2dcf80a3f2b956663c547825320637054cfcb16a578b24d2b372c117bef986b290d9c6055293fe446aabc2eabff35010fdbc383317f847be6824b
+2f5426417db872ddde8c60fe4581ade5ca3eed7fd15e2ae1454639a8c283bd600ffa201da13af8471450eb850296b3d06b471d558c08f3f1e52e9b3cbafa70d7

IO/FFMPEG/vtkFFMPEGVideoSource.cxx

@@ -126,6 +126,12 @@ vtkFFMPEGVideoSource::vtkFFMPEGVideoSource()
   , FeedThreadId(-1)
   , FileName(nullptr)
 {
+  // changed from superclass
+  this->OutputFormat = VTK_RGB;
+  this->FrameBufferBitsPerPixel = 24;
+  this->NumberOfScalarComponents = 3;
+  this->FrameBufferRowAlignment = 4;
+
   this->Internal = new vtkFFMPEGVideoSourceInternal;
 }