move Stb lib to 3rdparty folder

cmake/Modules/FindStb.cmake

@@ -0,0 +1,30 @@
+# Distributed under the OSI-approved BSD 3-Clause License.
+# Copyright Stefano Sinigardi
+
+#.rst:
+# FindStb
+# ------------
+#
+# Find the Stb include headers.
+#
+# Result Variables
+# ^^^^^^^^^^^^^^^^
+#
+# This module defines the following variables:
+#
+# ``Stb_FOUND``
+#   True if Stb library found
+#
+# ``Stb_INCLUDE_DIR``
+#   Location of Stb headers
+#
+
+include(${CMAKE_ROOT}/Modules/FindPackageHandleStandardArgs.cmake)
+include(${CMAKE_ROOT}/Modules/SelectLibraryConfigurations.cmake)
+
+if(NOT Stb_INCLUDE_DIR)
+  find_path(Stb_INCLUDE_DIR NAMES stb_image.h PATHS ${Stb_DIR})
+endif()
+
+find_package_handle_standard_args(Stb DEFAULT_MSG Stb_INCLUDE_DIR)
+mark_as_advanced(Stb_INCLUDE_DIR)