use a single function to parse non-IDAT chunks

use separate function to read IHDR
decode only the header in spng_get_ihdr()

This reverts commit a8fb4d0ce5eed524a2db31d1ce47b176b6c4ebf9.

prev_scanline is not accessed for filter types 0 and 1

Switch to the progressive decode and FILE* read API,
remove some unnecessary error checks

the last pass is the bottom half of the full image and
does not require deinterlacing

Do not install files if used as static subproject

See https://github.com/mesonbuild/meson/issues/2550

refactor the testsuite to pass the FILE* to the libraries
remove unused testsuite code, cleanup

The palette is not dynamically allocated,
so there is no chance of buffer overflow,
you just get...

the "white heavy checkmark" character is rendered as a green checkbox,
who would have thought.

improve formatting, wording, etc

it was off by one and it is the absolute lowest signed value anyway

decode_init() as a separate function is unnecessary

more work towards progressive read support
move init/finish/deinit code from decode_image()
refa...

the next version will be a minor release

Set ctx->stored.plte bit field when reading PLTE chunk from disk

Without this bit field being set the color palette of an image read
from disk cannot be accessed.

this controls the use of the target_clones
function attribute