[libpng16] Changed png_memcpy to C assignment where appropriate. Changed all

those uses of png_memcpy that were doing a simple assignment to assignments
(all those cases where the thing being copied is a non-array C L-value.)
Added some error checking to png_set_*() routines and removed the
reference to the non-exported function png_memcpy() from example.c. Fixed
the Visual C 64-bit build - it requires jmp_buf to be aligned, but it had
become misaligned.

example.c

diff --git a/example.c b/example.c
index 015a0aa..ad188e5 100644
--- a/example.c
+++ b/example.c
@@ -718,7 +718,7 @@
     * png_progressive_combine_row() passing in the new row and the
     * old row, as demonstrated above.  You can call this function for
     * NULL rows (it will just return) and for non-interlaced images
-    * (it just does the png_memcpy for you) if it will make the code
+    * (it just does the memcpy for you) if it will make the code
     * easier.  Thus, you can just do this for all cases:
     */