Use 10-bit packed preview format and add OCIO LUT regression tests

Renderer / preview:
- Switch the preview/display readback format from F16 to packed
  10-bit RGBA (PixelFormat::U10) to halve GPU->CPU/IPC bandwidth
  while keeping 10-bit panel precision.
- Add U10 support to VideoParams, FFmpeg/OIIO/OCIO utility mappings,
  OpenGL (GL_RGB10_A2), Vulkan (VK_FORMAT_A2B10G10R10_UNORM_PACK32),
  and plugin bit-depth lookups.
- Update preview autocacher comment to reflect the new behavior.

OCIO LUT tests:
- Add four E2E-style ColorLutNode gtests that drive a SolidGenerator
  -> OCIOLutNode graph through NodeTraverser and compare resulting
  pixels on the CPU. They cover forward/inverse transforms and verify
  that switching LUT direction and LUT file updates both the processor
  and the output pixels.

Cleanup:
- Remove a leftover SolidGenerator::Value debug fprintf.
- Capture render worker stderr in RenderWorkerFootageTest for better
  diagnostics.

app/render/previewautocacher.cpp

@@ -688,9 +688,11 @@ RenderTicketWatcher *PreviewAutoCacher::RenderFrame(Node *node,
 		rvp.AddCache(frame_cache);
 	} else {
 		// Preview/display frames are rendered at reduced precision to cut the
-		// GPU->CPU readback and IPC transfer bandwidth in half. The internal
-		// render pipeline stays F32/ACEScg; only the final preview copy is F16.
-		rvp.force_format = PixelFormat::F16;
+		// GPU->CPU readback and IPC transfer bandwidth. The internal render
+		// pipeline stays F32/ACEScg; the final preview copy is packed 10-bit
+		// RGBA (4 bytes/pixel) to preserve 10-bit panel precision while halving
+		// bandwidth compared to F16.
+		rvp.force_format = PixelFormat::U10;
 		rvp.force_channel_count = VideoParams::kRGBAChannelCount;
 	}