Fix unsound bitmap buffers vec instead of a dropped vec's pointer

[samsieber]

Here's my initial attempt at resolving the unsound bitmap buffer method (issue #174), quick and dirty. I'm using it locally in a fork, but this submission is just to give ideas about how you'd like this tackled.

The nice part is that there are method we can use to write code that works equally well on a &[u8] (from the native code path) and a Vec (from the wasm code path):

• .as_ref() on a &[u8] and a Vec<u8> return the same thing: &[u8]
• Vec::from(buffer) works on both a &[u8] and Vec<u8>

[ajrcarey]

Discussed and resolved as part of #174.