RFR: 8274699: Certain blend modes cannot be set from CSS

[Marius Hanl]

This PR fixes a bug where the blend mode will be falsely recognized as a color and therefore won't be set. 
Also a ClassCastException is thrown (The converter expects a String, but gets a Color).

Note: There is another similar bug but I can't reproduce this (Tried on 18-ea+3).
https://bugs.openjdk.java.net/browse/JDK-8268657

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Commit messages:
 - 8274699: Blend mode won't be detected and converted as color anymore

Changes: https://git.openjdk.java.net/jfx/pull/640/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=640&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274699
  Stats: 21 lines in 2 files changed: 9 ins; 6 del; 6 mod
  Patch: https://git.openjdk.java.net/jfx/pull/640.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/640/head:pull/640

PR: https://git.openjdk.java.net/jfx/pull/640