Fix "all|no frames are keyframes" settings.

Documentation says: "if kmin == 0, then key-frame insertion is disabled;
and if kmax == 0, then all frames will be key-frames."
Reading this, you'd expect that if kmax == 0, then with any kmin <= 0
all frames will be key-frames. But actually the kmin <= 0 test is caught
first and you get the opposite (no keyframes but the first). You'd have
instead to set kmax == 0 and any value kmin > 0, which is absolutely
counter-intuitive (reversing order).
Moreover kmax == 1 has no valid kmin (kmin == 1 conflicts with the
`kmax > kmin` rule and kmin == 0 conflicts with `kmin >= kmax / 2 + 1`).
So it should be considered an exception too.

Instead I propose this new logic:
- kmax == 1 means that all frames are keyframes (you are explicitly
  requesting a keyframe every 1 frame at most, i.e. all frames).
- kmax == 0 means no keyframes (you ask for a keyframe every 0 frames,
  i.e. never).
This is more "logical" language-wise, and also does not involve any
conflicts about what if both kmax and kmin are 0, since now a single
property value is meaningful for the 2 exceptional cases.

Change-Id: Ia90fb963bc26904ff078d2e4ef9f74b22b13a0fd

man/gif2webp.1

@@ -1,5 +1,5 @@
 .\"                                      Hey, EMACS: -*- nroff -*-
-.TH GIF2WEBP 1 "June 23, 2016"
+.TH GIF2WEBP 1 "January 25, 2017"
 .SH NAME
 gif2webp \- Convert a GIF image to WebP
 .SH SYNOPSIS
@@ -68,8 +68,9 @@ Specify the minimum and maximum distance between consecutive key frames
 some key frames into the output animation as needed so that this criteria is
 satisfied.
 .br
-A 'kmin' value of 0 will turn off insertion of key frames. A 'kmax' value of 0
+A 'kmax' value of 0 will turn off insertion of key frames. A 'kmax' value of 1
-will result in all frames being key frames.
+will result in all frames being key frames. 'kmin' value is not taken into
+account in both these special cases.
 Typical values are in the range 3 to 30. Default values are kmin = 9,
 kmax = 17 for lossless compression and kmin = 3, kmax = 5 for lossy compression.
 .br

src/mux/anim_encode.c

@@ -129,14 +129,13 @@ static void SanitizeEncoderOptions(WebPAnimEncoderOptions* const enc_options) {
     DisableKeyframes(enc_options);
   }
 
-  if (enc_options->kmin <= 0) {
+  if (enc_options->kmax == 1) {  // All frames will be key-frames.
-    DisableKeyframes(enc_options);
-    print_warning = 0;
-  }
-  if (enc_options->kmax <= 0) {  // All frames will be key-frames.
     enc_options->kmin = 0;
     enc_options->kmax = 0;
     return;
+  } else if (enc_options->kmax <= 0) {
+    DisableKeyframes(enc_options);
+    print_warning = 0;
   }
 
   if (enc_options->kmin >= enc_options->kmax) {

src/webp/mux.h

@@ -21,7 +21,7 @@
 extern "C" {
 #endif
 
-#define WEBP_MUX_ABI_VERSION 0x0107        // MAJOR(8b) + MINOR(8b)
+#define WEBP_MUX_ABI_VERSION 0x0108        // MAJOR(8b) + MINOR(8b)
 
 //------------------------------------------------------------------------------
 // Mux API
@@ -430,9 +430,10 @@ struct WebPAnimEncoderOptions {
                         // frames in the output. The library may insert some key
                         // frames as needed to satisfy this criteria.
                         // Note that these conditions should hold: kmax > kmin
-                        // and kmin >= kmax / 2 + 1. Also, if kmin == 0, then
+                        // and kmin >= kmax / 2 + 1. Also, if kmax <= 0, then
-                        // key-frame insertion is disabled; and if kmax == 0,
+                        // key-frame insertion is disabled; and if kmax == 1,
-                        // then all frames will be key-frames.
+                        // then all frames will be key-frames (kmin value does
+                        // not matter for these special cases).
   int allow_mixed;      // If true, use mixed compression mode; may choose
                         // either lossy and lossless for each frame.
   int verbose;          // If true, print info and warning messages to stderr.