FIX: Add workaround for cases where button presses can be missed (ISXB-491)

[graham-huws]

Description

Fix for ISXB-491. The current design of the input system package relies on the following method for checking if a button has been pressed on the current frame: Check the current value of the button, and compare to the final value of the previous frame. The issue with this approach is that presses can be missed if they happened very quickly, or there was some form of lag making the update take longer than intended. Whether a button is pressed is only properly evaluated when asked, so if we don't check every input coming in, some can be missed.

Changes made

This PR seeks to solve this in a very simplistic way - check every value that affects ButtonControls and record their pressed state at the time the change comes in. Current belief is that a "clever" solution to this issue would require a fairly extensive rewrite of how the input system works. Performance comparison data can be viewed on Sheet 2 here.

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.
• [ ] 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:

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