color: global LUT library, LUT error reporting, clamp + display fixes

- Add a global LUT library: user-configurable directories (new
  Preferences > LUT tab) scanned recursively for .cube/.3dl files; LUT
  node file pickers offer the library dirs as sidebar shortcuts via a
  'lut_library' input property handled by the param view bridge
- OCIOLutNode no longer fails silently: missing files, unsupported
  extensions and OCIO load errors are recorded in last_error() and
  surfaced in the status bar (input still passes through for rendering
  safety)
- ColorDialog: re-enable the display -> reference conversion using
  ColorProcessor::kInverse with a validity guard, and re-enable the
  Display tab in ColorValuesWidget; covered by a round-trip regression
  test proving the old OCIO inverse crash no longer occurs
- OCIOGradingTransformLinearNode: enforce the OCIO clampWhite >
  clampBlack invariant per frame in Value() so keyframed/connected
  values cannot produce invalid grading transforms, and constrain the
  white clamp UI minimum whenever the black clamp is static
- Regression tests for LUT extension checks, direction switching, node
  error reporting, LUT library scanning, display inverse round-trip and
  clamp enforcement

app/dialog/preferences/preferences.cpp

@@ -33,6 +33,7 @@
 #include "tabs/preferencesdisktab.h"
 #include "tabs/preferencesaudiotab.h"
 #include "tabs/preferenceskeyboardtab.h"
+#include "tabs/preferencesluttab.h"
 #include "window/mainwindow/mainwindow.h"
 
 namespace olive
@@ -60,6 +61,7 @@ PreferencesDialog::PreferencesDialog(MainWindow *main_window, int start_tab)
 		new PreferencesBehaviorTab(PreferencesBehaviorTab::kCategoryRendering),
 		tr("Rendering"));
 	AddTab(new PreferencesDiskTab(), tr("Disk"));
+	AddTab(new PreferencesLutTab(), tr("LUT"));
 	AddTab(new PreferencesKeyboardTab(main_window), tr("Keyboard"));
 
 	SetCurrentTab(start_tab);