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Automatically determine if variable row height support is needed, and if not use faster processing as per original SlickGrid code.
Revert "Automatically determine if variable row height support is needed, and if not use faster processing as per original SlickGrid code."

This reverts commit 4632986758f6091ad3c5922efbe880b3b1559f36.
Move row node cache into new 'cache' object. And create unit test for group rendering.
Fix bug with grouped cells being rendered due to cache not getting recalculated after rows count is changed
Optimization for row position cache.