activemap - add option to vpxenc to take an activemap

[GoogleCodeExporter]

I'd like to use activemap to improve performance, while transparent to the 
decoder that it was used.
Performance is currently not good, when using activemap, and having it exposed 
in vpxenc would allow it to be tested under more realistic conditions with 
arbitrary content.
A single activemap would suffice.  I suggest a raw 8 bit image with resolution 
equal to the activemap size.
The vpxenc option would supply the filename of the activemap, enabling the 
feature.

Mainly the performance focus should be quickly skipping blocks where the map 
value is 0.
Later additional options could expose other activemap threshholds, but those 
are not needed for now.

Original issue reported on code.google.com by [email protected] on 23 Dec 2010 at 10:00