Further activity masking changes:

Some further re-structuring of activity masking code.
Still has various experimental switches.
Supports a metric based on intra encode.
Experimental comparison against a fixed activity target  rather
than a frame average, for altering rd and zbin.

Overall the SSIM performance is similar  to TT's original
code but there is a much smaller PSNR hit of circa
0.5% instead of 3.2%

Change-Id: I0fd53b2dfb60620b3f74d7415e0b81c1ac58c39a