Drop-in compatibility

For spng_compat to become drop-in compatible the following additional features/functions have to be implemented on top of libspng:

• Error reporting with setjmp()
• Reading from a FILE*
• (Alpha) filler bytes (png_set_filler()).
• Various alpha blending modes (png_set_alpha_mode()).
• png_get_pixels_per_inch(), png_get_x/y_offset_inches() and fixed-point equivalents.
• Extensions: pCAL, sCAL, these require parsing of text.
• png_set_sig_bytes(), png_sig_cmp().
• low-level PNG writing functions: png_write_sig(), png_write_chunk(), etc.
• png_set_shift()
• png_set_invert_mono()
• png_set_compression_level() and similar functions.
• png_get_channels()
• png_set_rows()
• png_get/set_image_width/height/bit_depth/color_type/etc()
• png_get_(x/y)_pixels_per_meter()
• png_get_pixel_aspect_ratio(_fixed)()
• png_get_(x/y)_offset_(pixels/microns)()
• png_get_copyright(), png_get_header_ver(), png_get_header_version(), png_get_libpng_ver().
• png_permit_mng_features()
• png_set_user_limits(), etc.
• png_get_pHYs_dpi()