v1.21.8

Changes between v1.21.6 and v1.21.8:
a447060 throttle framerate on osx metal too when in background as catalina seems to have the same issue as previous versions had with gl
d6b7934 print a message if unable to initialize lua engine
cd29129 SDL backend: re-init controllers in Lua after reloading (ny kevinw)
dd223d6 disable dragging of html canvas (only seems to be an issue on ie)
95f58b0 fixes for newer emscripten