[lworld] RFR: C++ preparation for preview mode

[David Beaumont]

C++ changes for supporting preview mode *when* preview mode resources (with new location flags) are available.

At the moment, this code will operate on non-preview jimage files (1.0) and act as if no preview resources are available by virtue of the default value for missing attributes being zero (which matches location flags for "normal" entries).

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Commit messages:
 - C++ preparation for preview mode

Changes: https://git.openjdk.org/valhalla/pull/1609/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1609&range=00
  Stats: 326 lines in 8 files changed: 215 ins; 60 del; 51 mod
  Patch: https://git.openjdk.org/valhalla/pull/1609.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1609/head:pull/1609

PR: https://git.openjdk.org/valhalla/pull/1609