Simplified API: write-to-memory, overflow handling

This implements an API and provides a number of assist macros to allow an
application which uses the simplified API write to bypass stdio and write
directly to memory.

It also includes some warnings (png.h) and some check code to detect *possible*
overflow in the ROW_STRIDE and simplified image SIZE macros.  This disallows
image width/height/format that *might* overflow.  A quiet API change that limits
in-memory image size (uncompressed) to less that 4GByte and image row size
(stride) to less than 2GByte.

Signed-off-by: John Bowler <jbowler@acm.org>

scripts/symbols.def

@@ -249,3 +249,4 @@ EXPORTS
  png_set_check_for_invalid_index @242
  png_get_palette_max @243
  png_set_option @244
+ png_image_write_to_memory @245