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/framehashcache.cpp

@@ -433,6 +433,8 @@ bool FrameHashCache::SaveCacheFrame(const QString &filename,
 				fmt = QImage::Format_RGB888;
 			}
 			break;
+		case PixelFormat::U10:
+			break;
 		case PixelFormat::U16:
 			if (frame->channel_count() == VideoParams::kRGBAChannelCount) {
 				fmt = QImage::Format_RGBA64_Premultiplied;