renamed PIMPL::context to PIMPL::glxcontext

so we know which context this variable actually names
(NOT the gem::Context)
added member 'gemcontext' (for context sharing)
per display-string context-sharing

this uses a std::map to map display-strings to shared contexts
Closes https://sourceforge.net/p/pd-gem/bugs/235/

however, it is disabled for now (as it crashes).

TODO: free context when it is no longer used?
fixed copyright
removed unused code