Always include post-thumbnail-template and deprecate require_if_theme_supports()

[nacin]

require_if_theme_supports() was a nice idea at the time, but we've never found reason to leverage it. Let's remove it, and always include post-thumbnail-template.php.

As it is, some of these functions are useful when post thumbnails are not supported by the theme. (get_post_thumbnail_id(), for example). The whole file with these functions is under 100 lines.

Always including this file has the benefit of avoiding a sysopen call. opcode caching FTW. see #17112.

No plugins use require_if_theme_supports().

[nacin]

In [20610]:

Deprecate require_if_theme_supports(). Always require post-thumbnail-template.php. fixes #20556. fixes #20409.

[scribu]

Wait, plugins might not use require_if_theme_supports(), but themes do.

It's useful if you allow child themes to disable certain features.

[nacin]

I had searched themes.svn, but not very well. Looks like a few themes use it: ​https://www.google.com/search?q=require_if_theme_supports+site%3Athemes.svn.wordpress.org.

Happy with keeping it in theme.php.

[jkudish]

un-deprecate require_if_theme_supports function

[nacin]

In [20642]:

Undeprecate require_if_theme_supports() for themes using it for legitimate reasons. see [20610]. props scribu, jkudish. fixes #20556.