API to disable loop filter about webp HOT 3 CLOSED

add an option in VP8Io to bypass_filtering

https://review.webmproject.org/1827

this is quite an advance feature, so better try to avoid multiplying the
functions at the "Simple API" level (decode.h) and make it available in
the 2nd-level one (decode_vp8.h) instead

Following are the relevant filtering options exposed in webp encoder (cwebp)

  -strong ................ use strong filter instead of simple.
  -f <int> ............... filter strength (0=off..100)
  -sharpness <int> ....... filter sharpness (0:most .. 7:least sharp)

There are two filter levels simple (default) and strong.
For each of these level, you can choose filter strength (0..100), with 0 
effectively means shutting off the filtering.