Remove Utilities/OpenGL module

- OpenGL library is now loaded at runtime.

Documentation/release/dev/remove-opengl-module.md

+# Remove OpenGL module
+
+VTK no longer provides the `VTK::opengl` module because VTK now loads an OpenGL library
+at runtime using `glad`.
+
+The CMake setting `OpenGL_GL_PREFERENCE` is no longer relevant because VTK does not look for
+OpenGL at compile time. On Linux distributions, the `glad` OpenGL loader always loads `libGL.so` at runtime.

Utilities/OpenGL/CMakeLists.txt

-include(vtkOpenGLOptions)
-
-if (OpenGL_GL_PREFERENCE STREQUAL "LEGACY")
-  set(use_legacy ON)
-else ()
-  if (POLICY CMP0072)
-    cmake_policy(GET CMP0072 opengl_policy)
-    if (opengl_policy STREQUAL "NEW")
-      set(use_legacy OFF)
-    endif ()
-  else ()
-    # Without the policy, only legacy is available.
-    set(use_legacy ON)
-  endif ()
-endif ()
-
-set(opengl_components)
-set(opengl_optional_components)
-if (VTK_USE_X)
-  # XXX(nvidia): It seems that on some Ubuntu installations with manually
-  # installed nVidia drivers, GLX is not found. Investigation is necessary.
-
-  if (NOT use_legacy)
-    list(APPEND opengl_optional_components GLX)
-    list(APPEND opengl_targets OpenGL::GLX)
-  endif ()
-endif ()
-if (VTK_OPENGL_USE_GLES)
-  list(APPEND opengl_components GLES3)
-  list(APPEND opengl_targets OpenGL::GLES3)
-else ()
-  list(APPEND opengl_components OpenGL)
-  if (VTK_USE_WIN32_OPENGL)
-    list(APPEND opengl_targets OpenGL::GL)
-  else ()
-    if (use_legacy)
-      list(APPEND opengl_targets OpenGL::GL)
-    else ()
-      list(APPEND opengl_targets OpenGL::OpenGL)
-    endif ()
-  endif ()
-endif ()
-if (VTK_OPENGL_HAS_EGL)
-  list(APPEND opengl_components EGL)
-  list(APPEND opengl_targets OpenGL::EGL)
-endif ()
-
-if (VTK_OPENGL_HAS_OSMESA AND NOT vtk_can_do_onscreen)
-  vtk_module_third_party_external(
-    PACKAGE OSMesa
-    TARGETS OSMesa::OSMesa)
-else ()
-  cmake_policy(PUSH)
-  if (POLICY CMP0072)
-    cmake_policy(SET CMP0072 NEW) # prefer GLVND
-  endif ()
-
-  vtk_module_third_party_external(
-    PACKAGE             OpenGL
-    COMPONENTS          ${opengl_components}
-    OPTIONAL_COMPONENTS ${opengl_optional_components}
-    TARGETS             ${opengl_targets}
-    INCLUDE_DIRS        OPENGL_INCLUDE_DIR
-    LIBRARIES           OPENGL_LIBRARIES)
-
-  cmake_policy(POP)
-
-  if ("OpenGL::OpenGL" IN_LIST opengl_targets AND
-      NOT TARGET OpenGL::OpenGL AND
-      "OpenGL::GLX" IN_LIST opengl_targets AND
-      TARGET OpenGL::GLX)
-    message(FATAL_ERROR
-      "It appears as though OpenGL was found, but a GLVND-based install was "
-      "expected and it was not found. Please install GLVND libraries.")
-  endif ()
-
-  if (TARGET OpenGL::OpenGL AND NOT use_legacy)
-    set(opengl_preference "GLVND")
-  elseif (UNIX AND TARGET OpenGL::GL AND use_legacy)
-    set(opengl_preference "LEGACY")
-  else ()
-    set(opengl_preference "")
-  endif ()
-  set_property(GLOBAL
-    PROPERTY
-      _vtk_opengl_preference "${opengl_preference}")
-
-  if (APPLE)
-    vtk_module_definitions(VTK::opengl
-      INTERFACE
-        GL_SILENCE_DEPRECATION)
-  endif ()
-
-  if (VTK_USE_OPENGL_DELAYED_LOAD)
-    vtk_module_link(VTK::opengl
-      INTERFACE
-        delayimp.lib)
-    vtk_module_link_options(VTK::opengl
-      INTERFACE
-        "/DELAYLOAD:opengl32.dll")
-  endif ()
-endif ()

Utilities/OpenGL/vtk.module

-NAME
-  VTK::opengl
-LIBRARY_NAME
-  vtkopengl
-THIRD_PARTY