stop emulator if error occurred during setup
emscripten - add handling for -env
support setting ENV variables
emscripten - workaround for undefined symbol: __sys_shutdown
non-working at this stage
Updated buildjsgl4es.sh to include new sdl platform files
more recent gl4es lib. Also subsumed gl4es props into env param
tidy up gl4es integration into emscripten build. Now builds with extensions