Fix #1243

[rhgb]

#1243 This issue is caused by:

1. cssTransforms props of GridItem is determined by combining two things in ReactGridLayout: the property cssTransforms, and the state mounted.
2. The mounted state is initially false, causing GridItem using offset and percentage for positioning initially, instead of transform.
3. The mounted state is set to true in componentDidMount. But because shouldComponentUpdate method doesn't check it, the GridItem rendered won't change, until another update is triggered.

This PR fixes this issue. I've checked that it works in my application.

[STRML]

Thanks!

[mario-jerkovic]

Is there any eta for this fix :)

[STRML]

Yes, it's in 1.0.0.