Dash After-Image implemented

[peypod]

Implementation for an After Image while Dashing. The "Dash Icon" is spawned at the player position every frame while dashing, which gives good coverage. Depending on changes to the tick-rate, a more nuanced effect with interpolation may be preferred. Icons fade out quickly. also works with Stealth; The Alpha of the Icon image is set lower if stealthed, giving a good effect imo.

Code is currently in player_behaviour.rs and player/mod.rs so it's close to the dash code; depending on further features (like hit-sparks), refactoring it to a file named something like player_vfx.rs may make for a cleaner code structure.

[odecay]

One thing I noticed on this is that the chill effect does not apply to the afterimage sprites, If that is desired you could implement in a follow up PR.