fix/feat: Screen resizing artifacts + expose screen events!

[eonarheim]

===:clipboard: PR Checklist :clipboard:===

• 📌 issue exists in github for these changes
• 🔬 existing tests still pass
• 🙈 code conforms to the style guide
• 📐 new tests written and passing / old tests updated with new scenario(s)
• 📄 changelog entry added (or not needed)

==================

• Fixed issue where play button was hidden when going fullscreen mode
• Fixed issue where screen resizing caused artifacts on the loading screen
• Added new ex.Screen.events
  • screen.events.on('resize', (evt) => ) Will emit when the screen is resized
  • screen.events.on('fullscreen', (evt) => ) Will emit when the screen is changed into browser fullscreen mode
  • screen.events.on('pixelratio', (evt) => ) Will emit when the screen's pixel ratio changes (moving from a hidpi screen to a non, or vice versa)