Updated the near-lossless level mapping.

Updated the near-lossless level mapping and make it correlated to lossy
quality i.e 100 => minimum loss (in-fact no-loss) and the visual-quality loss
increases with decrease in near-lossless level (quality) till value 0.

The new mapping implies following (PSNR) loss-metric:
-near_lossless 100: No-loss (bit-stream same as -lossless).
-near_lossless  80: Very very high PSNR (around 54dB).
-near_lossless  60: Very high PSNR (around 48dB).
-near_lossless  40: High PSNR (around 42dB).
-near_lossless  20: Moderate PSNR (around 36dB).
-near_lossless   0: Low PSNR (around 30dB).

Change-Id: I930de4b18950faf2868c97d42e9e49ba0b642960

man/cwebp.1

@@ -1,5 +1,5 @@
 .\"                                      Hey, EMACS: -*- nroff -*-
-.TH CWEBP 1 "July 30, 2014"
+.TH CWEBP 1 "Feb 4, 2015"
 .SH NAME
 cwebp \- compress an image file to a WebP file
 .SH SYNOPSIS
@@ -222,7 +222,7 @@ Encode the image without any loss.
 Use near-lossless image preprocessing. This option adjusts pixel values
 to help compressibility, but has minimal impact on the visual quality.
 It triggers lossless compression mode automatically.
-Range is 0 (no preprocessing, the default) to 100.
+Range is 0 (maximum preprocessing) to 100 (no preprocessing, the default).
 .TP
 .BI \-hint " string
 Specify the hint about input image type. Possible values are: