Fix OpenGL include dependency.

This reverts commit 35a3e376de773d6d98cf5cd8af1b1e74d2629d97.

Instead of adding OPENGL_INCLUDE_DIR to every module that depends on
vtkRenderingOpenGL, we add OPENGL_INCLUDE_DIR as one of the
vtkRenderingOpenGL_INCLUDE_DIRS. That way, any module depending on
vtkRenderingOpenGL will automatically know where to find gl.h since gl.h is
needed by the header files for classes in vtkRenderingOpenGL.

Change-Id: I6675c2b522e15b3fc6f7a80c28f91c56660b671e
Merge topic 'fix_opengl_dependency' into master

ad146c5d Fix OpenGL include dependency.