Make opengl optional

[topheg]

Hello, This PR makes openGL an optional dependency of libcitygml :

• It adds a CMake option "LIBCITYGML_USE_OPENGL" (defaults to ON) to enable or disable the use of OpenGL
• An abstract class named TesselatorBase is created
• The Tesselator class inherits from TesselatorBase. It is only compiled if LIBCITYGML_USE_OPENGL is set to ON
• The methods that took a "Tesselator&" as a parameter now take a "Tesselator *". The "tesselate" boolean has been removed: if you don't want to tesselate, just pass a nullptr for the TesselatorBase parameter.
• Users can implements other tesselator by inheriting from TesselatorBase if the still want tesselation but don't want to depend on OpenGL

[topheg]

Hello, Having tested this branch for a while now, I think it is ready to be merged if you find it useful