We were compiling `src/loaders/external_png` but using the header from
`src/loaders/png`, leading to `-Wodr` warnings.
Remove unused bundled lodepng code since we use `external_png` (libpng).
Whitespace changes are from upstream, will attempt PR'ing a cleanup there for
future updates.
ThorVG is a platform-independent portable library for drawing vector-based
scene and animation.
Co-authored-by: Rémi Verschelde <rverschelde@gmail.com>
