internal/ui: enable some functions even on fullsceen
internal/ui: enable some functions even on fullsceen

This is thanks to a GLFW v3.3.8 change:
https://github.com/glfw/glfw/commit/9950cc52df60c21eda0896ff14b2357fe1f7b114

Updates #1590

hajimehoshi/ebiten,hajimehoshi/ebiten