Revert the removal of FindOpenGL2

It seems to have problems in some cases.

cmake/COPYING-CMAKE-SCRIPTS

+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions
+are met:
+
+1. Redistributions of source code must retain the copyright
+   notice, this list of conditions and the following disclaimer.
+2. Redistributions in binary form must reproduce the copyright
+   notice, this list of conditions and the following disclaimer in the
+   documentation and/or other materials provided with the distribution.
+3. The name of the author may not be used to endorse or promote products 
+   derived from this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
+IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
+INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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+THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

cmake/FindOpenGLES2.cmake

+# - Try to find OpenGLES2
+# Once done this will define
+#
+#  OPENGLES2_FOUND           - system has OpenGLES2
+#  OPENGLES2_INCLUDE_DIR     - the GLES2 include directory
+#  OPENGLES2_LIBRARY	    - the GLES2 library
+#
+# Copyright (c) 2012, George Kiagiadakis <kiagiadakis.george@gmail.com>
+#
+# Redistribution and use is allowed according to the terms of the BSD license.
+# For details see the accompanying COPYING-CMAKE-SCRIPTS file.
+
+find_path(OPENGLES2_INCLUDE_DIR GLES2/gl2.h)
+find_library(OPENGLES2_LIBRARY NAMES GLESv2)
+
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(OpenGLES2 DEFAULT_MSG OPENGLES2_INCLUDE_DIR OPENGLES2_LIBRARY)
+
+mark_as_advanced(OPENGLES2_INCLUDE_DIR OPENGLES2_LIBRARY)
+