Fix chunk data memory leak when decoding PNG

[miere43]

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Description

Added missing PngChunk.Dispose call to avoid memory leak in PngDecoderCore

All relevant information in issue #2080

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[tocsoft]

@miere43 Thanks a lot for this fix.

Turns out most of our test images suffer from this same issue, I have merged your fix (but not your tests) into #2082 so your fix has been merged via that PR instead so I'm going to close this one.