Add handling for indexed-color image type in WP Image Editor

[pbearne]

A new functionality has been added to the WP Image Editor class to check if an image is indexed-color encoded. This property is considered when saving a resized image to preserve its original image type. This reduces instances where the resized image filesize would end up larger than the original. A test to verify smaller file size for resized images has also been included.

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

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