Fix Flipped for libpng Extension Saving

[RobertBColton]

I ran onto this while working on #1725 and hugar also mentioned it to me and on the forums at the same time. The libpng extension was not respecting the flipped parameter when saving. The reason is because we were flipping both the source read and destination write indices, where one obviously undoes the other. We only need to flip one of the indexes, not both. The one we should flip is the destination write index, since it is bounded by height, where the source read index is bounded by fullheight.

[JoshDreamland]

It may be time to consider deleting this file. We have smarter, more up-to-date PNG read/write logic elsewhere throughout the project.

[codecov]

Codecov Report

Merging #1728 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1728      +/-   ##
==========================================
- Coverage   24.67%   24.66%   -0.01%     
==========================================
  Files         166      166              
  Lines       16590    16588       -2     
==========================================
- Hits         4094     4092       -2     
  Misses      12496    12496

Impacted Files	Coverage Δ
.../Universal_System/Extensions/libpng/libpng-ext.cpp	93.61% <100%> (-0.27%)	⬇️

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[RobertBColton]

Yes, we can take a look at that later. It seems we are actually using the libpng-util you speak of, but doing additional cropping and flipping. We have a universal helper for that already, image_flip. I don't feel like getting too involved with this right now, we can come back to it later.