RFR: 8260360: IGV: Short name of combined nodes is hidden by background color

[Koichi Sakata]

This pull request enables the short name of combined nodes readable. 

At present those node are painted out with black because their OutputSlot color is null. So this pull request sets the original color to the OutputSlot.

I tested the following scenario manually:

- Open a graph, then enable "Simplify graph" (as described in the bug report). The result is the following images. There are two graphs. One is black and white only graph, the other is colored graph.

<img width="540" alt="スクリーンショット 2021-05-18 16 25 03" src="https://user-images.githubusercontent.com/60008/118609442-b36bf780-b7f5-11eb-86a4-109b4f11b857.png">
<img width="540" alt="スクリーンショット 2021-05-18 16 25 33" src="https://user-images.githubusercontent.com/60008/118609451-b666e800-b7f5-11eb-85e0-1e3604109a8a.png">

-------------

Commit messages:
 - Set the original color to OutputSlot

Changes: https://git.openjdk.java.net/jdk/pull/4082/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4082&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260360
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4082.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4082/head:pull/4082

PR: https://git.openjdk.java.net/jdk/pull/4082