Fix issue in PNG identify method to skip "uninteresting" chunks, incl…

[flew2bits]

…uding sBIT.

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Description

Add a default clause in the PNG Identify method to skip over "uninteresting" chunks so that the data is properly read.
This fixes issue #2018

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[flew2bits]

The call to SkipChunkDataAndCrc was in the wrong location to fix the problem. I've moved it to a location that should fix the problem.

[flew2bits]

I moved the call to SkipChunkDataAndCrc back to the default switch clause based on how other chunks are handled in the switch statement. It's not wrapped in a check to see if the identification is for color metadata only, also based on other clauses.

[brianpopow]

I have found an png with a sBit chunk:

Would that reproduce the described issue in #2018? If so, it would be good to have a unit test covering this issue. See PngDecoderTests for an example howto do that.

[flew2bits]

Unfortunately that image doesn't exhibit the problem. I'll see if I can put one together that can be used for testing.

[JimBobSquarePants]

I'm actually happy to push this in without the offending image. As a performance optimization alone it's worth the addition.

@flew2bits if you could supply an image at some point that would be really great.

[brianpopow]

I'm actually happy to push this in without the offending image. As a performance optimization alone it's worth the addition.

@flew2bits if you could supply an image at some point that would be really great.

@JimBobSquarePants should we milestone this for 2.0.1, also?

[JimBobSquarePants]

@brianpopow Yeah... Let's get it merged now.