glretrace: Destroy Context only once via glXDestroyContext

Handling adapted from retrace_wglDeleteContext

Observed crash on Linux/X11 (valgrind log)

==20050== Invalid read of size 8
==20050==    at 0x42D65A: glretrace::Context::~Context() (glretrace_ws.cpp:122)
==20050==    by 0x5E4CCB: retrace_glXDestroyContext(trace::Call&) (glretrace_glx.cpp:159)
==20050==    by 0x429D1C: retrace::Retracer::retrace(trace::Call&) (retrace.cpp:119)
==20050==    by 0x41F83B: retrace::retraceCall(trace::Call*) (retrace_main.cpp:173)
==20050==    by 0x4216FB: retrace::RelayRunner::runLeg(trace::Call*) (retrace_main.cpp:324)
==20050==    by 0x4215ED: retrace::RelayRunner::runRace() (retrace_main.cpp:295)
==20050==    by 0x41FB8B: retrace::RelayRace::run() (retrace_main.cpp:462)
==20050==    by 0x41FD5A: retrace::mainLoop() (retrace_main.cpp:522)
==20050==    by 0x4204B2: main (retrace_main.cpp:781)
==20050==  Address 0xa9567e0 is 0 bytes inside a block of size 24 free'd
==20050==    at 0x4C2BADC: operator delete(void*) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==20050==    by 0x5E4CD3: retrace_glXDestroyContext(trace::Call&) (glretrace_glx.cpp:159)
==20050==    by 0x429D1C: retrace::Retracer::retrace(trace::Call&) (retrace.cpp:119)
==20050==    by 0x41F83B: retrace::retraceCall(trace::Call*) (retrace_main.cpp:173)
==20050==    by 0x4216FB: retrace::RelayRunner::runLeg(trace::Call*) (retrace_main.cpp:324)
==20050==    by 0x4215ED: retrace::RelayRunner::runRace() (retrace_main.cpp:295)
==20050==    by 0x41FB8B: retrace::RelayRace::run() (retrace_main.cpp:462)
==20050==    by 0x41FD5A: retrace::mainLoop() (retrace_main.cpp:522)
==20050==    by 0x4204B2: main (retrace_main.cpp:781)