Add optional Vulkan hardware acceleration for H.264 transcoding

[khamilowicz]

• Adds a native_acceleration: :never | :if_available option to Membrane.Transcoder that routes H.264 decode/encode through Membrane.VKVideo.Decoder / Membrane.VKVideo.Encoder when Vulkan hardware is present.
• Adds Membrane.Transcoder.vulkan_available?/0 — a public helper for callers that need to check GPU availability before building a pipeline.
• Handles NV12 pixel-format conversion around the Vulkan path (VK decoder outputs NV12; SWScale converters are inserted automatically when the surrounding formats differ).
• Keeps membrane_vk_video_plugin as an optional dependency — software fallback is unchanged when the dep is absent or native_acceleration: :never.

Test plan

• Run existing integration tests: mix test
• Run new Vulkan integration tests on a machine with a Vulkan-capable GPU: mix test --include vulkan
• Verify the updated examples/vp8_to_h264.exs produces valid output with and without native_acceleration: :if_available
• Confirm vulkan_available?/0 returns false on a machine without membrane_vk_video_plugin installed

🤖 Generated with Claude Code

[varsill]

It's kind of non-trivial to figure out how to determine if vulkan video extensions are available :D
There is no requirement to use pkg-config but also plain existence of vulkan pkg-config entry does not indicate that Vulkan Video Extensions are available (it's gets even worse - Vulkan Video Extensions might be present, but there might not be H264 encoding and decoding capabilities).
I think we should uncodintionally add an optional dependency to membrane_vk_video_plugin and rely on fact that user will install it only after checking that particular capabilities are delivered.

[varsill]

It would be good to assert on output file content as well

[varsill]

I am not sure I understand this part :D

[varsill]

I would use some other name for that flag than use_vulkan? - sth like use_hardware_acceleration?