change resizable="true" for html5 demos

[ninjamuffin99]

accommodates for #322

we have two options for setting proper fullscreen

1. changing the iframe / width stuff on the fly
   • this may have ramifications due to CORS policy stuff, where if we wanted to adjust the width/height of game from OUTSIDE of the iframe it might be funky!
2. allowing the window to be resized to fill the viewport

this PR does option 2, so now when making the iframe fullscreen (whether that's via in-game code, or from the site) it simply fills whatever window it's in.