Add option to invert rows in spng_encode_image

[tobybell]

This merge request is to share back something I implemented in a downstream copy of the project. No pressure to accept it; just sharing in case it's useful.

Previously, spng_encode_image would always interpret the img data pointer as pointing to image data in top-to-bottom order, despite some image data APIs storing image data in bottom-to-top order (most notably, OpenGL functions like glReadPixels).

This merge request adds a flag, SPNG_ENCODE_INVERT_ROWS to allow spng_encode_image to interpret the given image data pointer as containing rows in bottom-to-top order, instead of top-to-bottom. This allows saving PNG images directly from glReadPixels calls when using OpenGL, without an additional copy to reverse the row data.