AutoPolygon::findFirstNoneTransparentPixel() detected Y pos is 1 pixe…

[aismann]

…l to much.
e.g.: correct is {2,2} but returning {2,3}

Describe your changes

see diff.

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For each PR

• Add Copyright if it missed:
  - "Copyright (c) 2019-present Axmol Engine contributors (see AUTHORS.md)."

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  Optional:

  • I have checked readme and add important infos to this PR.
  • I have added/adapted some tests too.

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• I have checked readme and add important infos to this PR.
• I have added thorough tests.

Axmol 3.x ------------------------------------------------------------

For each 3.x PR

• Check the '#include "axmol.h"' and replace it with the needed headers.

[aismann]

@halx99
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