Intra block copy masking

The intra block copy method improves coding efficiency of still picture and video codecs by enabling patches of already decoded picture area to be used as prediction for later areas in the same picture. However, due to practical reasons block copying is typically restricted to use a certain predefined prediction block grid. This leads to suboptimal performance as especially in the case of high frequency content, like text and graphics, the sample prediction process typically copies some residue on the block boundaries in addition to the actual texture of interest. The proposed technique improves the intra block copy method by allowing the codec to clean up unwanted residual signal at the block boundaries.