Make several functions take &[T] instead of Vec[T]

The CSFML functions all copy the data given to them and do not take
ownership, so taking Vec<T> is unnecessary at best and an extra heap
allocation (for data created on the stack) at worst.

[breaking-change]
Merge pull request #44 from tomjakubowski/update-pixels-slice

Make several functions take &[T] instead of Vec[T]