Add Vulkan NV12 texture handle support for QVideoFrame/QML rendering

[Copilot]

Vulkan-decoded NV12 frames were exposed to Qt as if both planes shared the same importable texture handle, which breaks QML rendering and produces the green/noisy output shown in the issue. This change aligns the Vulkan frame export path with Qt's per-plane texture import expectations.

• Vulkan frame allocation

  • Configure FFmpeg Vulkan frames with AV_VK_FRAME_FLAG_DISABLE_MULTIPLANE so NV12 is backed by importable per-plane VkImages instead of a single multi-planar image.

• QVideoFrame handle plumbing

  • Add QAVVideoFrame::VulkanTextureHandle.
  • Map Vulkan-backed QAVVideoFrame instances to Qt's RhiTextureHandle path in QVideoFrame conversion.

• Per-plane Vulkan handle export

  • Introduce Vulkan-specific video buffer handling for Qt 6.
  • Export one VkImage handle per plane for Vulkan/NV12 frames instead of duplicating img[0].
  • Centralize Vulkan handle extraction and guard against invalid frame context / plane count mismatches.

• API contract

  • Document that QAVVideoFrame::handle() may return a QList<quint64> for multi-plane hardware frames.

• Coverage

  • Extend the existing integration test path to assert Vulkan handle extraction when Vulkan decoding is active.

QList<quint64> textures;
for (int plane = 0; plane < planeCount; ++plane)
    textures.push_back(quint64(vk_frame->img[plane]));
return QVariant::fromValue(textures);

• Fixes Add vulkan handle to QAVVideoFrame #722

[valbok]

@copilot It does not work:
Run like this:

$ QSG_RHI_BACKEND=vulkan ./qml_video ../../tests/auto/integration/testdata/DHC0413_CreaseOrNot.mp4 
Missing Vulkan image for plane 1 of 2
No Vulkan image handles in the frame 1.13447
Missing Vulkan image for plane 1 of 2
No Vulkan image handles in the frame 1.13447
Missing Vulkan image for plane 1 of 2
No Vulkan image handles in the frame 1.13447

[Copilot]

@copilot It does not work:
Run like this:

...

Addressed in aa13c2c. The Vulkan frames init now overrides FFmpeg's prefilled multi-planar NV12 format with the per-plane fallback formats before av_hwframe_ctx_init(), instead of falling back to img[0] when plane 1 is missing.

[valbok]

@copilot this now returns vk_frame->img[0] and vk_frame->img[1] images, but rendering is 100% green

[Copilot]

Addressed in f28e197. The Vulkan handle path is now gated on the FFmpeg frame and Qt QRhi using the same Vulkan device; otherwise it stops exposing raw VkImage handles so Qt falls back to the mapped-memory upload path instead of sampling foreign-device images.

[valbok]

@copilot this produces green artifacts, but recognizable image

[Copilot]

Addressed in 686b2ae. When the Vulkan-specific mapTextures() path fails, it now suppresses the generic textureHandle() fallback so Qt drops to the mapped-memory upload path instead of sampling the broken raw Vulkan handles that produced the green artifacts.