RFR: 8275345: RasterFormatException when drawing a tiled image made of non-writable rasters

[Martin Desruisseaux]

This commit fixes a `RasterFormatException` thrown when `Graphics2D.drawRenderedImage(…)` is invoked with an image containing tiles that are instances of `Raster` but not `WritableRaster`.

https://bugs.openjdk.java.net/browse/JDK-8275345

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Commit messages:
 - 8275345: RasterFormatException thrown when drawing a tiled image made of non-writable rasters.

Changes: https://git.openjdk.java.net/jdk/pull/7337/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7337&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275345
  Stats: 218 lines in 2 files changed: 209 ins; 8 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7337.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7337/head:pull/7337

PR: https://git.openjdk.java.net/jdk/pull/7337