RFR: 8251032: Colors with texture background look different with Alpha Com…

[Denis Konoplev]

…posite

Set missing sourceAlphaBlendFactor

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Commit messages:
 - 8251032: Colors with texture background look different with Alpha Composite

Changes: https://git.openjdk.java.net/lanai/pull/86/files
 Webrev: https://webrevs.openjdk.java.net/?repo=lanai&pr=86&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8251032
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/lanai/pull/86.diff
  Fetch: git fetch https://git.openjdk.java.net/lanai pull/86/head:pull/86

PR: https://git.openjdk.java.net/lanai/pull/86