Improve Linux/Mesa support

[jonpepler]

⚠️ I've made heavy use of AI for this. The changes work for my purposes so I'm offering them back, but I can't test on other platforms.

Fix Hullcam frame capture on Linux / Mesa (all-white JPEGs)

Hullcam frames arrive as all-white JPEGs on native Linux installs (tested: Steam Deck SteamOS 3.x, native KSP 1.12.5 Linux). Metadata (camera name, speed, altitude) flows correctly; only the rendered pixels are broken.

Three bugs in TrackingCamera.SendCameraImage() compound on Linux where they happen to be masked on Windows/D3D:

1. Graphics.CopyTexture(RT, Tex2D) followed by AsyncGPUReadback.Request(Tex2D, 0) relies on the driver implicitly synchronising the GPU→GPU copy before the readback. Mesa's OpenGL driver doesn't guarantee that ordering; readback returns without error but the destination Texture2D contains uninitialised pixels - which read back as pure white in ARGB32.
2. Task.Run + ContinueWith running LoadRawTextureData and EncodeToJPG off the main thread. Unity's texture APIs are main-thread-only per Unity's docs. D3D tolerates it; Mesa doesn't.
3. Capturing request.GetData<byte>() into a Task closure. The returned NativeArray<byte> is only valid inside the readback callback's synchronous scope; once it returns the backing allocation is disposed. The .ContinueWith thread-pool task reads freed memory.

Diagnostic evidence

A debug build logging AsyncGPUReadback.Request(TargetCamRenderTexture, 0, …) directly (skipping the Graphics.CopyTexture intermediate) showed 100% of readbacks returning hasError=true on Mesa. The upstream copy-to-Tex2D-first path sidesteps that specific error but loses the pixel data a different way, hence all-white frames rather than no frames at all.

Fix

Replace the async dance with the canonical synchronous capture pattern:

var previousActive = RenderTexture.active;
RenderTexture.active = TargetCamRenderTexture;
_texture2D.ReadPixels(new Rect(0, 0, _texture2D.width, _texture2D.height), 0, 0);
_texture2D.Apply();
RenderTexture.active = previousActive;
_jpgTexture = _texture2D.EncodeToJPG();
// Only the gRPC send goes off-thread: pure I/O, no Unity APIs.
Task.Run(() => GrpcClient.SendCameraTextureAsync(payload));

Synchronous ReadPixels blocks the main thread for ~1–2 ms per Hullcam frame at 768x768.

Compatibility

Strictly better-behaved on all platforms, not Linux-specific. Windows/D3D users see identical results with no regression.

Testing

• Native Linux (Steam Deck, SteamOS 3): confirmed working live video feeds
• No Windows install available for regression testing; happy to coordinate with someone who has one