RFR: 8258202: Lanai: Buffered image loses its shape after clicking on Alpha Composite option

[Denis Konoplev]

Fill alpha channel for RGB BufferedImages.

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Commit messages:
 - 8258202: Lanai: Buffered image loses its shape after clicking on Alpha Composite option

Changes: https://git.openjdk.java.net/lanai/pull/153/files
 Webrev: https://webrevs.openjdk.java.net/?repo=lanai&pr=153&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8258202
  Stats: 55 lines in 2 files changed: 13 ins; 33 del; 9 mod
  Patch: https://git.openjdk.java.net/lanai/pull/153.diff
  Fetch: git fetch https://git.openjdk.java.net/lanai pull/153/head:pull/153

PR: https://git.openjdk.java.net/lanai/pull/153