Get a list of palette base colors that are used in the above and
left blocks, referred to as "color cache". For each cache color,
signal if it is present in current block's palette, so that we
don't need to transmit their raw values.

When palette-delta-encoding is enabled, compression is improved
by 2% on keyframe and 1% overall for the screen_content testset.

Change-Id: I4cb027f1904aa9d0ab1c8f00ea9ee34bf5f16234