Add Qt6 multimedia backend plugin (Qt 6.4+)

[Copilot]

Qt 6.4 introduced a new QPlatformMediaPlugin / QPlatformMediaIntegration API for custom multimedia backends, replacing the Qt5 QMediaServiceProviderPlugin model. This PR wires MDK into that new framework while keeping the Qt5 path intact.

New Qt6 plugin layer (qt6/)

• mdkplatformmediaplugin — QPlatformMediaPlugin entry point; responds to key "mdk" via Q_PLUGIN_METADATA(IID QPlatformMediaPlugin_iid)
• mdkmediaintegration — QPlatformMediaIntegration factory; creates the player and a default QPlatformVideoSink
• mdkplayercontrol — QPlatformMediaPlayer implementation wrapping MDK's Player; all MDK callbacks dispatch to the main thread via QMetaObject::invokeMethod

Video output

Two paths, tried in order:

1. Host-copy (MDK_ABI): player_.renderVideo() → getVideoFrame() → plane-by-plane memcpy into a QVideoFrame → QVideoSink::setVideoFrame(). Zero GL overhead for software-decoded content.
2. FBO readback (hardware / non-MDK_ABI): render into a dedicated QOffscreenSurface + QOpenGLContext, read back via QOpenGLFramebufferObject::toImage(), push as QVideoFrameFormat::Format_RGBA8888.

Build system

• .qmake.conf and mdkmediaservice.pro conditionally select PLUGIN_TYPE=multimedia / sources for Qt6, mediaservice / original sources for Qt5.
• Qt6 plugin installs to <Qt>/plugins/multimedia/; Qt5 unchanged at <Qt>/plugins/mediaservice/.
• QT += multimedia-private opengl added for Qt6 builds.

Plugin metadata

mdkmediaservice.json key renamed "mdkservice" → "mdk" (required for Qt6 discovery; Qt5 uses the "Services" field and is unaffected by the key name). Existing users relying on QT_MULTIMEDIA_PREFERRED_PLUGINS=mdkservice will need to update to mdk.

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[wang-bin]

• before qt6.10, QPlatformMediaIntegration::createSome() returns QMaybe<QPlatformSome*>, since qt6.10 returns q23::expected<QPlatformSome*,QString/*error*/>

[wang-bin]

before qt6.10, QPlatformMediaIntegration::createSome() returns QMaybe<QPlatformSome*>, since qt6.10 returns q23::expected<QPlatformSome*,QString/error/>

[Copilot]

before qt6.10, QPlatformMediaIntegration::createSome() returns QMaybe<QPlatformSome*>, since qt6.10 returns q23::expected<QPlatformSome*,QString/error/>

Updated MDKMediaIntegration to use QMaybe on Qt 6.4–6.9 and q23::expected on Qt 6.10+, committed in 7db0a1d.

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