Frozen column fixes for dyanically locking and unlocking columns

[sonny-chan]

Description

If a user is attempting to unlock columns dynamically while the grid is scrolled to the right - the original locked column header cells will not be visible due to the transform that was applied to it during the "locking".

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/adazzle/react-data-grid/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (check one with "x")

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior? (You can also link to an open issue here)
Attempting to unlock columns while scrolled causes the the original locked column header cells to not be visible.

What is the new behavior?
Unlocking columns will now show the original header cells so you can read the column name.

Does this PR introduce a breaking change? (check one with "x")

[ ] Yes
[x] No

If this PR contains a breaking change, please describe the impact and migration path for existing applications: ...

Other information:

[coveralls]

Changes Unknown when pulling 86a6c8b on sc-frozen-column-fixes into ** on master**.