RFR: 8364687: Enable headless with -Dglass.platform=headless

[Kevin Rushforth]

On Tue, 5 Aug 2025 07:45:34 GMT, Clément de Tastes <duke at openjdk.org> wrote:

> 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.

Reviewer: @johanvos

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PR Comment: https://git.openjdk.org/jfx/pull/1864#issuecomment-3154855116