This PR fixes a memory leak in dnd code in the `MouseHandler` of a `Scene`. The memory leak occurs after calling `startFullDrag()`. The `fullPDRTmpTargetWrapper` is then populated but never cleared. Fix is to call `clear()` on the `fullPDRTmpTargetWrapper`, very similar to https://github.com/openjdk/jfx/pull/448 ([JDK-8264330](https://bugs.openjdk.java.net/browse/JDK-8264330)).
------------- Commit messages: - 8279214: Memory leak in Scene after dragging a cell Changes: https://git.openjdk.org/jfx/pull/899/files Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=899&range=00 Issue: https://bugs.openjdk.org/browse/JDK-8279214 Stats: 42 lines in 2 files changed: 42 ins; 0 del; 0 mod Patch: https://git.openjdk.org/jfx/pull/899.diff Fetch: git fetch https://git.openjdk.org/jfx pull/899/head:pull/899 PR: https://git.openjdk.org/jfx/pull/899
