dev-qt/qtwayland: make compositor APIs off-by-default optional

[a17r]

Would have preferred a clean split but wayland-server depends on wayland-client in the buildsys.

• Drop IUSE vulkan: Merge with 'compositor' instead. This absolves us from having to make a default conflicting REQUIRED_USE with compositor.
• Drop IUSE X: Unused, broken upstream since 2021 and removed in Qt6. Quoting upstream from commit d4a7faff: "These plugins were created to allow testing/debugging of QtWayland- based compositors on desktops that did not support Wayland natively."

PS: Not runtime tested yet.

[Chiitoo]

Did not get to testing yet either, but no objections from me after just a look-see.

Thank you!

[Pesa]

No objections.

[a17r]

Of course, the decision to drop IUSE=vulkan leads to the following issues:

FAILURES
dev-qt/qtwayland
  NonsolvableDepsInStable: version 5.15.11: nonsolvable depset(depend) keyword(~arm) stable profile (default/linux/arm/17.0) (70 total): solutions: [ =dev-qt/qtgui-5.15.11*:5=[vulkan] ]
  NonsolvableDepsInStable: version 5.15.11: nonsolvable depset(depend) keyword(~hppa) stable profile (default/linux/hppa/17.0) (20 total): solutions: [ =dev-qt/qtgui-5.15.11*:5=[vulkan], dev-util/vulkan-headers ]
  NonsolvableDepsInStable: version 5.15.11: nonsolvable depset(rdepend) keyword(~arm) stable profile (default/linux/arm/17.0) (90 total): solutions: [ =dev-qt/qtgui-5.15.11*:5=[vulkan] ]

Maybe just mask qtwayland:5[compositor] on these arches in addition to vulkan.

[a17r]

Yup, down to this after cleaning up obsolete package.use.mask entries in ::gentoo:

dev-qt/qtwayland
  MissingRemoteId: missing <remote-id type="kde-invent">qt/qt</remote-id> (inferred from URI 'https://invent.kde.org/qt/qt/')
  NonsolvableDepsInStable: version 5.15.11: nonsolvable depset(depend) keyword(~hppa) stable profile (default/linux/hppa/17.0) (20 total): solutions: [ =dev-qt/qtgui-5.15.11*:5=[vulkan], dev-util/vulkan-headers ]
  NonsolvableDepsInStable: version 5.15.11: nonsolvable depset(rdepend) keyword(~hppa) stable profile (default/linux/hppa/17.0) (20 total): solutions: [ =dev-qt/qtgui-5.15.11*:5=[vulkan] ]