Add a shield effect and boost cap % consumption on collision

[jukibom]

This is to discourage but not totally eliminate collisions as a mechanism for quick U-turns while providing visual feedback to a collision (they can be very minor).

Potentially make the cap % loss a factor of the change in vector as a result of that collision - such that minor scrapes take ~1% of cap while full reverses eat, say, ~50%. But I'm not entirely sure how to detect that ...