Fixed: A dragged view was constrained inside a plaform window bounds.

For example, in CPTableView before this change, dragged rows could not move up if the table view was near the top of the platform window.

Fixes #1877
Test TableTest/ViewBasedCib/ and drag up the third row.
Merge pull request #1879 from cacaodev/CPDragServer-fixed-constrained-inside-bounds

Fixed: A dragged view was constrained inside a plaform window bounds.