FIX: Fixed a performance issue with many objects using multiple action maps [ISXB-573]

[AlexTyrer]

Description

Avoid costly (and pointless) iterations over all messages / objects / maps when we don't need to.

Issue: https://jira.unity3d.com/browse/ISXB-573

This codepath can be very expensive when there are a lot of messages to send to lots of objects with maps - much of the cost is in dereferencing handle.Target in DeferredResolutionOfBindings() which involves taking a mutex lock.

Further, once the lock is taken for each object, ResolveBindingsIfNecessary() is called for each of that object's InputActionMaps which in the general case does nothing (when there is nothing to do).

In the supplied (ridiculous to show the point) test project with 900 objects, each with 6 maps receiving 4500 mouse move messages this leads to:

• over 21 million calls to ResolveBindingsIfNecessary() that do nothing
• over 4 million mutex locks in resolving handle.Target in DeferredResolutionOfBindings()
• frame times over 1 second!

Changes made

This change adds a static InputActionMap.s_NeedToResolveBindings that is set when we know that at least one InputActionMap has changes that need resolving. This is cleared when a resolve pass has been completed.

With this change the frame time in the example only increases by about 2-3ms when the mouse is moving.

Checklist

Before review:

• [x] Changelog entry added.
  • Explains the change in Changed, Fixed, Added sections.
  • For API change contains an example snippet and/or migration example.
  • FogBugz ticket attached, example ([case %number%](https://issuetracker.unity3d.com/issues/...)).
  • FogBugz is marked as "Resolved" with next release version correctly set.
• [x] Tests added/changed, if applicable.
  • Functional tests Area_CanDoX, Area_CanDoX_EvenIfYIsTheCase, Area_WhenIDoX_AndYHappens_ThisIsTheResult.
  • Performance tests.
  • Integration tests.
• [x] Docs for new/changed API's.
  • Xmldoc cross references are set correctly.
  • Added explanation how the API works.
  • Usage code examples added.
  • The manual is updated, if needed.

During merge:

• [ ] Commit message for squash-merge is prefixed with one of the list:
  • NEW: ___.
  • FIX: ___.
  • DOCS: ___.
  • CHANGE: ___.
  • RELEASE: 1.1.0-preview.3.

After merge:

• [ ] Create forward/backward port if needed. If you are blocked from creating a forward port now please add a task to ISX-1444.

[ekcoh]

@AlexTyrer Seems to be conflicts in changelog that needs to be resolved before merge.

[AlexTyrer]

@AlexTyrer Seems to be conflicts in changelog that needs to be resolved before merge.

Thanks - I've resolved this now!