Activity masking modulation

[jmvalin]

Figure out how to modulate the effect of activity masking based on whether the block/superblock/whatever has texture vs edges.

[tmatth]

So far the best results (for ntt-short) have been with switching off activity masking for 8x8 blocks for higher resolution input (i.e. >= 1024*768), where I would guess 8x8 blocks are more likely to represent edges. Results: https://pastebin.mozilla.org/8839490