Add more context to split_the_query filter

[spacedmonkey]

Current the split_the_query filter has the current WP_Query instance as context. However to calculate the value you need the value variables.

• $old_request
• $limits
• $fields
• $q ( query arguments ).

It is worth noting that $this->request can be received by WP_Query instance and $wpdb->posts can be received from global $wpdb.

With this extra context, will be make the filter much more useful.

Trac ticket: https://core.trac.wordpress.org/ticket/59514

[spacedmonkey]

Ping @tillkruess for review.

[tillkruess]

Huge +1. No downsides.