WP Oembed in Multisite fails when embedded post comes from a different site of a network.

[imath]

Steps to reproduce:

1/ Create a post in main site.
2/ Create a new site
3/ Go the dashboard of this site
4/ Create a new post and paste in it the permalink of the post created at step 1.

This happens when the post we're trying to embed comes from a different site. I've added some unit tests to illustrate the issue. Without the patch, these two unit tests are failing:

• test_wp_filter_pre_oembed_result_multisite_sub_othersub
• test_wp_filter_pre_oembed_result_multisite_sub_main

[ocean90]

@swissspidy How does 40673.patch​ look to you?

[swissspidy]

Just did two minor tweaks in 40673.diff​. I think this is good to go.

[swissspidy]

In 41600:

Embeds: Improve performance when embedding a post on Multisite.

After [37798], this fixes embeds coming from a different site in the network.

Props imath.
Fixes #40673. See #36767.

[ocean90]

The use of ms_is_switched() doesn't seem to be right, see #38253 for a previous case.

[swissspidy]

In 41606:

Embeds: Maintain switched state when embedding a post on Multisite.

Props bor0.
Fixes #40673.

[ocean90]

The current blog isn't restored if $post_id is false.

[ocean90]

In 41634:

Embeds: In switched state, restore previous state if no post ID is found.

Fixes #40673.