The class is virtual and has one subclass, SubsampledYUVFrame, which
is used by the VP9 decoder to return a single frame. The
output_to_bitmap(Bitmap&) function can be used to set pixels on an
existing bitmap of the correct size to the RGB values that
should be displayed. The to_bitmap() function will allocate a new bitmap
and fill it using output_to_bitmap.
This new class also implements bilinear scaling of the subsampled U and
V planes so that subsampled videos' colors will appear smoother.
