[Core] Process all changed keys in one scan loop, deprecate QMK_KEYS_PER_SCAN

[KarlK90]

For Reviewers: The changeset is huge as many keyboards where adjusted in the process.
The important changes to core functionality are in separate and small commits. I have taken great care to not break any existing keyboard, hopefully this was successful.

Description

Short version

• Move matrix processing from keyboard_task() into seperate matrix_task()
• Process all key changes in one scan loop if there are changes in the key matrix
  • Deprecate QMK_KEYS_PER_SCAN (Allow multiple process_record() calls per scan #2022) which previously allowed to process more than one key in a single scan
  • Make debounce() algorithms signal changes in the debounced "cooked" keyboard matrix via a return value (will be a separate PR)
• Add LEGACY_MATRIX_SCAN bodge and info message for custom matrix scanning implementations that do not signal matrix changes (will be a separate PR)

Long version

This PR moves the core matrix processing from keyboard_task() into a separate matrix_task(). Processing all key changes of the matrix is also done in one scan loop if and only if there are key changes to be processed. This lazy evaluation of key changes is done by making the debounce() algorithms signal changes in the debounced "cooked" keyboard matrix via a return value. Previously the processing of more than one key change per scan loop could be enabled by setting QMK_KEYS_PER_SCAN to a number like 4 or 5 which was introduced in #2022 and I guess as a precaution not enabled for other keyboards. As a temporary bodge custom keyboard matrix implementations get a config flag LEGACY_MATRIX_SCAN which activates a info message like this:

Size impacts

ARM: +220 Bytes

❯ ./util/size_regression.sh -d maintenance/keyboard-scan-task planck/rev6:default
Size:    53368, delta:    +32 -- 9f6e1703bc Process the complete matrix in one scan loop
Size:    53336, delta:   +188 -- 7d5d9c3472 Remove QMK_KEYS_PER_SCAN option
Size:    53148, delta:     +0 -- 168a631720 Add support for MacroCat Keyboard (#17480)

AVR: -154 Bytes

❯ ./util/size_regression.sh -d maintenance/keyboard-scan-task crkbd:default
Size:    26254, delta:    +60 -- 9f6e1703bc Process the complete matrix in one scan loop
Size:    26194, delta:   -214 -- 7d5d9c3472 Remove QMK_KEYS_PER_SCAN option
Size:    26408, delta:     +0 -- 168a631720 Add support for MacroCat Keyboard (#17480)

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Issues Fixed or Closed by This PR

• (Hopefully) Reduce code complexity by refactoring the matrix loop into a separate task
• Make QMK_KEYS_PER_SCAN the default behavior as this benefits some users and won't hurt others
• Clean-up some matrix_scan() API and use its potential for lazy evaluation of the matrix
• Remove dead code

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[zvecr]

Remove deprecated matrix_is_modified() dead code

This should really be tackled in its own PR.

[drashna]

Doesn't appear to break anything, at the very least. been running it for a full day now, and appears to work fine.

[KarlK90]

@zvecr I have dropped the commit and openend #15349 that removes yet another unused method.

[stale]

Thank you for your contribution!
This pull request has been automatically marked as stale because it has not had activity in the last 45 days. It will be closed in 30 days if no further activity occurs. Please feel free to give a status update now, or re-open when it's ready.
For maintainers: Please label with awaiting review, breaking_change, in progress, or on hold to prevent the issue from being re-flagged.

[tzarc]

Deferred to Q3 cycle due to conflicts.

[ecerulm]

@KarlK90 , is this PR still alive?

[KarlK90]

@ecerulm currently on hold until the rp2040 support is finished

[alex-ong]

Thanks for the merge; does QMK now send multiple keys in one report? or is that waiting for #12686?

[KarlK90]

No QMK still only sends one key change per report.