Add custom viewport color correction (sRGB/OCIO), bypassing HdxColorCorrectionTask

Hydra's HdxColorCorrectionTask rendered prims black in OCIO mode and could
corrupt the GlfDrawTarget bind stack on failure (skipping Unbind), blacking
out every later frame including sRGB. Replace it with our own GL post-process:
the scene renders linear (RGBA16F) and is corrected by a fullscreen shader --
linear->sRGB encode, or OCIO via the OCIO 2.1 GPU API (GpuShaderDesc plus
uploaded 1D/3D LUT textures). OCIO build failures fall back to sRGB (never
black) and USD diagnostics are routed to the app log.

- core: ViewportColorCorrector + ApplyViewportColorCorrection in UsdSceneRenderer
- utils: OcioConfigParser enumerates displays/views/colorspaces/looks from $OCIO
- ui: gear-menu OCIO controls (ViewportTile) + per-viewport persistence (ViewportPanel)
- Application: point $OCIO at the bundled ACES 1.2 config
- CMake: link/copy OpenColorIO, download ACES 1.2 config; plus hdCycles build
  config (disable OpenVDB/Embree, fix TBB/OpenSubdiv/Imath dirs, exclude CRT DLLs)
- main: pre-flight plugin DLL load check to skip plugins with missing deps

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/ui/ViewportTile.h

@@ -26,9 +26,14 @@ struct ViewportTileSettings {
     float       bgColorG         = 0.15f;
     float       bgColorB         = 0.15f;
     int         bboxMode         = 0;     ///< BBoxMode cast to int
-    bool        ambientLightOnly = false;
+    bool        ambientLightOnly = true;
     bool        domeLightEnabled = false;
-    int         shadingMode      = 0;     ///< ShadingMode cast to int (0 = SmoothShaded)
+    int         shadingMode          = 0; ///< ShadingMode cast to int (0 = SmoothShaded)
+    int         colorCorrectionMode  = 1; ///< ColorCorrectionMode cast to int (1 = sRGB)
+    std::string ocioDisplay;
+    std::string ocioView;
+    std::string ocioColorSpace;
+    std::string ocioLook;
 };
 
 /// Named orthographic view directions.
@@ -178,6 +183,13 @@ private:
     // ── Orthographic view ────────────────────────────────────────────────────
     OrthoView m_orthoView = OrthoView::None;
 
+    // ── OCIO InputText edit buffers (per-tile, seeded on mode activation) ────
+    char m_ocioDisplayBuf[128]    = {};
+    char m_ocioViewBuf[128]       = {};
+    char m_ocioColorSpaceBuf[128] = {};
+    char m_ocioLookBuf[128]       = {};
+    bool m_ocioFieldsSynced       = false;
+
     // ── Per-frame interaction flags ──────────────────────────────────────────
     bool m_wasClickedThisFrame  = false;
     bool m_wasHoveredThisFrame  = false;