Misc. typos

Some are user facing. Some are in actual code. Most are in source comments. Also, please double check the changes in contrib/tools/pngfix.c
diff --git a/png.h b/png.h
index 3cc2bea..cba3e88 100644
--- a/png.h
+++ b/png.h
@@ -1193,7 +1193,7 @@
  *
  * png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_GAMMA_MAC);
  *    In this case the output is assumed to be something like an sRGB conformant
- *    display preceeded by a power-law lookup table of power 1.45.  This is how
+ *    display preceded by a power-law lookup table of power 1.45.  This is how
  *    early Mac systems behaved.
  *
  * png_set_alpha_mode(pp, PNG_ALPHA_STANDARD, PNG_GAMMA_LINEAR);
@@ -1240,7 +1240,7 @@
  *
  * When the default gamma of PNG files doesn't match the output gamma.
  *    If you have PNG files with no gamma information png_set_alpha_mode allows
- *    you to provide a default gamma, but it also sets the ouput gamma to the
+ *    you to provide a default gamma, but it also sets the output gamma to the
  *    matching value.  If you know your PNG files have a gamma that doesn't
  *    match the output you can take advantage of the fact that
  *    png_set_alpha_mode always sets the output gamma but only sets the PNG
@@ -1691,7 +1691,7 @@
  *           chunk will cause an error at this point unless it is to be saved.
  * positive: The chunk was handled, libpng will ignore/discard it.
  *
- * See "INTERACTION WTIH USER CHUNK CALLBACKS" below for important notes about
+ * See "INTERACTION WITH USER CHUNK CALLBACKS" below for important notes about
  * how this behavior will change in libpng 1.7
  */
 PNG_EXPORT(88, void, png_set_read_user_chunk_fn, (png_structrp png_ptr,
@@ -2239,7 +2239,7 @@
  * to specifying "NEVER", however when "AS_DEFAULT" is used for specific chunks
  * it simply resets the behavior to the libpng default.
  *
- * INTERACTION WTIH USER CHUNK CALLBACKS:
+ * INTERACTION WITH USER CHUNK CALLBACKS:
  * The per-chunk handling is always used when there is a png_user_chunk_ptr
  * callback and the callback returns 0; the chunk is then always stored *unless*
  * it is critical and the per-chunk setting is other than ALWAYS.  Notice that
@@ -2658,7 +2658,7 @@
  * The simplified API hides the details of both libpng and the PNG file format
  * itself.  It allows PNG files to be read into a very limited number of
  * in-memory bitmap formats or to be written from the same formats.  If these
- * formats do not accomodate your needs then you can, and should, use the more
+ * formats do not accommodate your needs then you can, and should, use the more
  * sophisticated APIs above - these support a wide variety of in-memory formats
  * and a wide variety of sophisticated transformations to those formats as well
  * as a wide variety of APIs to manipulate ancillary information.
@@ -3133,7 +3133,7 @@
     * than or equal to the original value.
     *
     * If the function returns false and *memory_bytes was not changed an error
-    * occured during write.  If *memory_bytes was changed, or is not 0 if
+    * occurred during write.  If *memory_bytes was changed, or is not 0 if
     * 'memory' was NULL, the write would have succeeded but for the memory
     * buffer being too small.  *memory_bytes contains the required number of
     * bytes and will be bigger that the original value.