Improve WPF Garbage Collection Performance

If you're using SharpGL in a WPF application, and you're using RenderContextType.FBO, then when the OpenGLControl responds to its DispatchTimer's Tick event (see the timer_Tick method in OpenGLControl.xaml.cs under the OpenGL.WPF folder) it will call the static HBitmapToBitmapSource(IntPtr) method (see BitmapConversion.cs). To prevent possible memory problems when too much is going and and garbage collection isn't fast enough, the HBitmapToBitmapSource method includes a call to GC.Collect() in a finally block that occurs with every call.

I have found this to cause too much of a slow-down with fast changing scenes, and to speed things up, I've made the following changes: I added a "static int gcCount = 0;" and a "static readonly int MaxCyclesBeforeGC = 50;" to the BitmapConversion class. Then, in the HBitmapToBitmapSource method's finally block have the following instead of the GC.Collect() call:

```
gcCount++;
if (gcCount > MaxCyclesBeforeGC)
{
gcCount = 0;
GC.Collect();
}
```

You can play around with the value for MaxCyclesBeforeGC, but NOT calling GC.Collect() with every cycle makes for a much smoother result and performing the garbage collection every so often seems to keep any memory problems in check.

Just a friendly suggestion.