RFR: 8364687: Enable headless with -Dglass.platform=headless [v2]

[Clément de Tastes]

> Basically allow -Dglass.platform=headless to enable headless, not only -Dglass.platform=Headless.
> This makes it consistent with other options (macosx, windows, linux, gtk and ios) as well was less error-prone for enabling it.

Clément de Tastes has updated the pull request incrementally with one additional commit since the last revision:

  remove additional space

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1864/files
  - new: https://git.openjdk.org/jfx/pull/1864/files/069d9615..91d6076d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1864&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1864&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/1864.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1864/head:pull/1864

PR: https://git.openjdk.org/jfx/pull/1864