add_theme_support from theme's should not be eclusionary

[webdevmattcrom]

If a theme defines an array for defining thumbnail support, like so: add_theme_support('post-thumbnails', 'posts'), then plugins cannot override it, meaning any plugin that creates a custom post type and wants it to utilize a featured image cannot do that with add_theme_support. A workaround has been posted here: ​http://wordpress.stackexchange.com/questions/23839/using-add-theme-support-inside-a-plugin

But there should be the ability to simply add more thumbnail support in a standard fashion.

[SergeyBiryukov]

Duplicate of #22080.