force "-pass 6" if -psnr or -size is used but -pass isn't.

This is to prevent users shooting in the foot using -psnr or
-size alone and not getting the expected result.

Change-Id: I67a3289e4ec0a2a813c98807f2ec5e600f52dc63

examples/cwebp.c

@@ -972,6 +972,11 @@ int main(int argc, const char *argv[]) {
                       " encoding. Ignoring this option!\n");
     }
   }
+  // If a target size or PSNR was given, but somehow the -pass option was
+  // omitted, force a reasonable value.
+  if (config.target_size > 0 || config.target_PSNR > 0) {
+    if (config.pass == 1) config.pass = 6;
+  }
 
   if (!WebPValidateConfig(&config)) {
     fprintf(stderr, "Error! Invalid configuration.\n");

man/cwebp.1

@@ -1,5 +1,5 @@
 .\"                                      Hey, EMACS: -*- nroff -*-
-.TH CWEBP 1 "December 14, 2015"
+.TH CWEBP 1 "February 11, 2016"
 .SH NAME
 cwebp \- compress an image file to a WebP file
 .SH SYNOPSIS
@@ -118,16 +118,20 @@ or without alpha).
 .BI \-size " int
 Specify a target size (in bytes) to try and reach for the compressed output.
 Compressor will make several pass of partial encoding in order to get as
-close as possible to this target.
+close as possible to this target. If both \fB\-size\fP and \fB\-psnr\fP
+as used, \fB\-size\fP value will prevail.
 .TP
 .BI \-psnr " float
 Specify a target PSNR (in dB) to try and reach for the compressed output.
 Compressor will make several pass of partial encoding in order to get as
-close as possible to this target.
+close as possible to this target. If both \fB\-size\fP and \fB\-psnr\fP
+as used, \fB\-size\fP value will prevail.
 .TP
 .BI \-pass " int
 Set a maximum number of passes to use during the dichotomy used by
 options \fB\-size\fP or \fB\-psnr\fP. Maximum value is 10, default is 1.
+If options \fB\-size\fP or \fB\-psnr\fP were used, but \fB\-pass\fP wasn't
+specified, a default value of '6' passes will be used.
 .TP
 .B \-af
 Turns auto\-filter on. This algorithm will spend additional time optimizing