frame_superres: Add members to AV1_COMMON

Adds the superres scale numerator, width, and height to the AV1_COMMON
struct for later use in encoding and decoding. This value will also be
added to the uncompressed frame header.

A #define statement is also added to represent the denominator - 16.

Change-Id: I4acec39ef3c67fa9f7118dac28daec3a2d734ed5
2 files changed