(Pre)process functions for nested hook suggestions are not invoked reliably

Problem/Motivation

This issue is composed of two aspects:

• when processing the theme registry nested hook suggestion are not always processed in the same order;
• when nested hook suggestions are processed in the intended order (more generic suggestions first), not all preprocess functions are invoked.

Example

function mytheme_preprocess_links(&$variables) {
  // ...
}
function mytheme_preprocess_links__entity(&$variables) {
  // ...
}
function mytheme_preprocess_links__entity__node(&$variables) {
  // ...
}

Depending on the runtime conditions when the registry is processed, theme('links__entity__node') might end up in one of these two cases:

• mytheme_preprocess_links__entity__node() and mytheme_preprocess_links__entity() are invoked;
• mytheme_preprocess_links__entity__node() and mytheme_preprocess_links() are invoked.

I'd expect all three preprocess functions to be invoked.

Proposed resolution

• Ensure that nested hook suggestions are always processed in a predictable order when processing the theme registry.
• Merge (pre)process and include data so that the whole hierarchy is traversed when dealing with the most specific suggestion.

Remaining tasks

• Validate the proposed solution
• Write a patch
• Review it

User interface changes

None

API changes

This will most definitely result in a behavior change, which could be considered a BC-breaking change. OTOH the current behavior can be very tricky, as the theme may behave differently on each theme registry rebuild.

Data model changes

None

Release notes snippet

TBD