Interplay MVE: Implement frame format 0x06

This implements the 0x06 frame format for Interplay MVE movies. The
format is relatively simple. The video data consists of two parts:

16 bits per 8x8 block movement data
a number of 8x8 blocks of pixel data

For each 8x8 block of pixel data the movement data is consulted. There
are 3 possible meanings of the movement data:
* zero     : copy the 8x8 block from the pixel data
* negative : copy the 8x8 block from the previous frame from an offset
             determined by the actual value of the entry -0xC000.
* positive : copy the 8x8 block from the current frame from an offset
             determined by the actual value of the entry -0x4000

Decoding happens in two passes, in the fist pass only new pixeldata is
copied, during the second pass data is copied from the previous and
current frames.

The codec expects that the current frame being decoded to still has the
data from 2 frames ago on it when decoding starts.

Signed-off-by: Hein-Pieter van Braam <hp@tmm.cx>