Improve resizing for indexed PNG images in WP Image Editor Imagick

[nosilver4u]

This does the following to improve resizing of indexed PNG images:

1. Added a method to get the PNG color type and pixel depth directly from the original image.
2. Get the original number of colors in an indexed PNG prior to scaling the image.
3. Preserve the tRNS chunk if an indexed PNG also has transparency.
4. Use quantizeImage() to reduce the number of colors in the image after scaling.
5. In cases where quantizeImage() converts the colorspace to grayscale, set png:format=png8 to force ImageMagick to output an indexed PNG.

Trac ticket: https://core.trac.wordpress.org/ticket/36477

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