Made the vertical flip of frames optional.

[tyronx]

When writing a frame via OpenTKs GL.ReadPixels(0, 0, width, height, PixelFormat.Bgra, PixelType.UnsignedByte, videoBuffer);

my frame seems to be already correctly oriented and sharpavi turns it upside down again. So here's a few simple getter/setter to make that part configurable.