look for python2 before python for default .emscripten

emscripten requires python 2 but some systems have multiple versions of
python installed and the 'python' binary will correspond to python 3. We
now explicitly look for a python2 first when setting up a default
.emscripten config file.
lots of SDL_GL_GetProcAddress fixes

These fixes are enough to enable us to start running Cogl based
applications via our sdl backend
Merge pull request #1117 from rib/wip/cogl

Enable simple Cogl based apps by fixing some SDL_GL_GetProcAddress issues