update http links

- prefer https

- metadataworkinggroup.org/com seem to be offline; the web archive link
  was obtained from exiftool: https://exiftool.org/TagNames/MWG.html

- fix kramdown link, rubyforge has been gone a long time

- fix png/zlib links

Bug: webp:544
Bug: b/202302177
Change-Id: Id69de4553e7baf00393f12a2c1acb262443a1a93

src/utils/huffman_encode_utils.c

@@ -161,7 +161,7 @@ static void SetBitDepths(const HuffmanTree* const tree,
 // especially when population counts are longer than 2**tree_limit, but
 // we are not planning to use this with extremely long blocks.
 //
-// See http://en.wikipedia.org/wiki/Huffman_coding
+// See https://en.wikipedia.org/wiki/Huffman_coding
 static void GenerateOptimalTree(const uint32_t* const histogram,
                                 int histogram_size,
                                 HuffmanTree* tree, int tree_depth_limit,

src/utils/quant_levels_dec_utils.c

@@ -30,7 +30,7 @@
 
 #define DFIX 4           // extra precision for ordered dithering
 #define DSIZE 4          // dithering size (must be a power of two)
-// cf. http://en.wikipedia.org/wiki/Ordered_dithering
+// cf. https://en.wikipedia.org/wiki/Ordered_dithering
 static const uint8_t kOrderedDither[DSIZE][DSIZE] = {
   {  0,  8,  2, 10 },     // coefficients are in DFIX fixed-point precision
   { 12,  4, 14,  6 },

src/utils/utils.c

@@ -23,7 +23,7 @@
 // alloc/free etc) is printed. For debugging/tuning purpose only (it's slow,
 // and not multi-thread safe!).
 // An interesting alternative is valgrind's 'massif' tool:
-//    http://valgrind.org/docs/manual/ms-manual.html
+//    https://valgrind.org/docs/manual/ms-manual.html
 // Here is an example command line:
 /*    valgrind --tool=massif --massif-out-file=massif.out \
                --stacks=yes --alloc-fn=WebPSafeMalloc --alloc-fn=WebPSafeCalloc