Polaris 0.2.21 (Minor bug fixes and balances)

- Exit to main menu button added to pause menu.

- Removed a softlock in the "downed bridge" room in mission 2

- Lowered the fire speed increase given by alien screws, but increased the base fire rate of mission 2

- Teki charge length reduced from 0.9 seconds to 0.75 seconds

- Wall-slobber AI changes to prevent it only shooting once rather than twice

- Wall-slobber movement timer reduced from a random value 0.5-2 seconds to 0.5-1.5 seconds

- Boss 3 total hp reduced from 60 to 52

- Made the Radium Donut's hiding spot more obvious

- Untested Linux version