Emscripten build

Upgrade libpng to 1.6.55 #1055

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Run time

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Job	Run time
Win10 -> Emscripten, Release	21m 52s
Win10 -> Emscripten, Debug	20m 30s
MacOS -> Emscripten, Debug	12m 10s
Linux -> Emscripten, Release	15m 46s
Linux -> Emscripten, Debug	14m 56s
	1h 25m 14s