Enc: add a qmin / qmax range for quality factor

This is particularly useful for multi-pass search (but not only),
to prevent the search from going over or below a reasonable threshold.
E.g.: 'cwebp -qrange 50 80 ...' will prevent any unreasonable degradation.

new cwebp option: -qrange min max

Change-Id: I59f394533535fc20b6996bc0895f4301476d5eff

README

@@ -254,6 +254,8 @@ Options:
   -partition_limit <int> . limit quality to fit the 512k limit on
                            the first partition (0=no degradation ... 100=full)
   -pass <int> ............ analysis pass number (1..10)
+  -qrange <min> <max> .... specifies the permissible quality range
+                           (default: 0 100)
   -crop <x> <y> <w> <h> .. crop picture with the given rectangle
   -resize <w> <h> ........ resize picture (after any cropping)
   -mt .................... use multi-threading if available
@@ -294,6 +296,7 @@ Experimental Options:
   -af .................... auto-adjust filter strength
   -pre <int> ............. pre-processing filter
 
+
 The main options you might want to try in order to further tune the
 visual quality are:
  -preset