Build: update CMake to support OpenEXR 3

FindOpenEXR was updated to find new lib names and separate Imath. It's all
added to the list of OpenEXR include dirs and libs.

This keeps it compatible with both version 2 and 3 for now, and doesn't
require changes outside the find module.

Ref D14128

build_files/cmake/Modules/FindOpenEXR.cmake

@@ -33,14 +33,6 @@ ENDIF()
 # Old versions (before 2.0?) do not have any version string, just assuming this should be fine though.
 SET(_openexr_libs_ver_init "2.0")
 
-SET(_openexr_FIND_COMPONENTS
-  Half
-  Iex
-  IlmImf
-  IlmThread
-  Imath
-)
-
 SET(_openexr_SEARCH_DIRS
   ${OPENEXR_ROOT_DIR}
   /opt/lib/openexr
@@ -93,6 +85,24 @@ UNSET(_openexr_libs_ver_init)
 
 STRING(REGEX REPLACE "([0-9]+)[.]([0-9]+).*" "\\1_\\2" _openexr_libs_ver ${OPENEXR_VERSION})
 
+# Different library names in 3.0, and Imath and Half moved out.
+IF(OPENEXR_VERSION VERSION_GREATER_EQUAL "3.0.0")
+  SET(_openexr_FIND_COMPONENTS
+    Iex
+    IlmThread
+    OpenEXR
+    OpenEXRCore
+  )
+ELSE()
+  SET(_openexr_FIND_COMPONENTS
+    Half
+    Iex
+    IlmImf
+    IlmThread
+    Imath
+  )
+ENDIF()
+
 SET(_openexr_LIBRARIES)
 FOREACH(COMPONENT ${_openexr_FIND_COMPONENTS})
   STRING(TOUPPER ${COMPONENT} UPPERCOMPONENT)
@@ -111,6 +121,57 @@ ENDFOREACH()
 
 UNSET(_openexr_libs_ver)
 
+IF(OPENEXR_VERSION VERSION_GREATER_EQUAL "3.0.0")
+  # For OpenEXR 3.x, we also need to find the now separate Imath library.
+  # For simplicity we add it to the OpenEXR includes and libraries, as we
+  # have no direct dependency on Imath and it's simpler to support both
+  # 2.x and 3.x this way.
+
+  # Find include directory
+  FIND_PATH(IMATH_INCLUDE_DIR
+    NAMES
+      Imath/ImathMath.h
+    HINTS
+      ${_openexr_SEARCH_DIRS}
+    PATH_SUFFIXES
+      include
+  )
+
+  # Find version
+  FIND_FILE(_imath_config
+    NAMES
+      ImathConfig.h
+    PATHS
+      ${IMATH_INCLUDE_DIR}/Imath
+    NO_DEFAULT_PATH
+  )
+
+  # Find line with version, extract string, and format for library suffix.
+  FILE(STRINGS "${_imath_config}" _imath_build_specification
+       REGEX "^[ \t]*#define[ \t]+IMATH_VERSION_STRING[ \t]+\"[.0-9]+\".*$")
+  STRING(REGEX REPLACE ".*#define[ \t]+IMATH_VERSION_STRING[ \t]+\"([.0-9]+)\".*"
+         "\\1" _imath_libs_ver ${_imath_build_specification})
+  STRING(REGEX REPLACE "([0-9]+)[.]([0-9]+).*" "\\1_\\2" _imath_libs_ver ${_imath_libs_ver})
+
+  # Find library, with or without version number.
+  FIND_LIBRARY(IMATH_LIBRARY
+    NAMES
+      Imath-${_imath_libs_ver} Imath
+    NAMES_PER_DIR
+    HINTS
+      ${_openexr_SEARCH_DIRS}
+    PATH_SUFFIXES
+      lib64 lib
+    )
+  LIST(APPEND _openexr_LIBRARIES "${IMATH_LIBRARY}")
+
+  # In cmake version 3.21 and up, we can instead use the NO_CACHE option for
+  # FIND_FILE so we don't need to clear it from the cache here.
+  UNSET(_imath_config CACHE)
+  UNSET(_imath_libs_ver)
+  UNSET(_imath_build_specification)
+ENDIF()
+
 # handle the QUIETLY and REQUIRED arguments and set OPENEXR_FOUND to TRUE if
 # all listed variables are TRUE
 INCLUDE(FindPackageHandleStandardArgs)
@@ -119,13 +180,25 @@ FIND_PACKAGE_HANDLE_STANDARD_ARGS(OpenEXR  DEFAULT_MSG
 
 IF(OPENEXR_FOUND)
   SET(OPENEXR_LIBRARIES ${_openexr_LIBRARIES})
-  # Both include paths are needed because of dummy OSL headers mixing #include <OpenEXR/foo.h> and #include <foo.h> :(
-  SET(OPENEXR_INCLUDE_DIRS ${OPENEXR_INCLUDE_DIR} ${OPENEXR_INCLUDE_DIR}/OpenEXR)
+  # Both include paths are needed because of dummy OSL headers mixing
+  # #include <OpenEXR/foo.h> and #include <foo.h>, as well as Alembic
+  # include <half.h> directly.
+  SET(OPENEXR_INCLUDE_DIRS
+    ${OPENEXR_INCLUDE_DIR}
+    ${OPENEXR_INCLUDE_DIR}/OpenEXR)
+
+  IF(OPENEXR_VERSION VERSION_GREATER_EQUAL "3.0.0")
+    LIST(APPEND OPENEXR_INCLUDE_DIRS
+      ${IMATH_INCLUDE_DIR}
+      ${IMATH_INCLUDE_DIR}/Imath)
+  ENDIF()
 ENDIF()
 
 MARK_AS_ADVANCED(
   OPENEXR_INCLUDE_DIR
   OPENEXR_VERSION
+  IMATH_INCLUDE_DIR
+  IMATH_LIBRARY
 )
 FOREACH(COMPONENT ${_openexr_FIND_COMPONENTS})
   STRING(TOUPPER ${COMPONENT} UPPERCOMPONENT)