A bit and a three bit literal have been added to the uncompressed frame
header for use by the frame superres experiment.

The bit is true if scaling is to be used, and is immediately followed by
a three bit literal that encodes the scale to use. If the first bit is
false, scaling is disabled and the scale factor numerator is set to the
denominator (ie. 1:1 scaling). No literal follows if scaling is
disabled.

The denominator has been defined as a constant 16. The literal is biased
by a defined constant of 8 - allowing fractions from 1/2 to 15/16
scaling in steps of 1/16 when scaling is used. Experimentation will be
needed to discover which of these are useful.

The bit and literal are immediately after the optional render_width and
render_height, so that the superres parameters can be written and read
just after the regular width and height, without interfering with the
render_size parameters.

This patch also adds an arbitrary write to make the scale 1:1, so as to
not trigger any scaling until it's ready.

Accompanying encode and decode helper functions are added.

Change-Id: I8caa6247c73f5c7f84ef1fde1e80eb9b20bde0e3