Use native image API for NS

Use native image API for NS

* configure.ac (NATIVE_IMAGE_API): Move above NS definitions.
(HAVE_NATIVE_IMAGE_API): Set for NS.
(HAVE_PNG, HAVE_JPEG, HAVE_GIF, HAVE_TIFF): Enable on NS builds.
* src/image.c (HAVE_NS): Fix a number of #if's so they no longer rely
on HAVE_NS.
(PIX_MASK_DRAW): Add for HAVE_NS so libpng support will compile.
(image_can_use_native_api):
(native_image_load): Add NS support.
(png_load):
(jpeg_load):
(tiff_load):
(gif_load): Remove NS specific definitions.
* src/nsimage.m (ns_can_use_native_image_api): New function.
* src/nsterm.h: (ns_can_use_native_image_api): New function.