Improve PNG parsing performance.

[RoosterDragon]

Switch on the filter once per row rather than once per byte. This allows each row to be processed with a much tighter loop.

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This makes the PNG parser 1.8x faster. When loading the RA mod it reduces time spent parsing PNGs from 4.4% to 2.4%.

[pchote]

Code changes look sensible, and doesn't obviously break anything that I could see in the loadscreens/main menu/map previews.

[Mailaender]

@OpenHV load time reduction

this: 2769 ± 28 ms
bleed: 2843 ± 4 ms

[Mailaender]

Changelog