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By not including the *T_impl.hh files in the corresponding *T.hh files,
but only providing explicit template declarations and instantiations for
the types we use them for, we can avoid recompilation of code that
(indirectly) depends on the *T.hh files, which turn out to be many.

This still leaves a lot of re-linking with libACG, but at least it's
much faster than before.

also removed some obsolete checks for compatibility profiles, which
prevented the proper use of the optimized picking variant when compat
profiles with version 3.2 or newer were present

removed chck for ARB_vertex_buffer_object as it has become part of opengl spec in version 1.5 and is not listed as extension by some drivers (e.g. all apple drivers)