More int type fixes

Just pulled in nightly got a whole bunch of errors like this:
```
error: cannot determine a type for this expression: cannot determine the type of this integer; add a suffix to specify the type explicitly
```

Not sure, may be this just a follow-up on rust-lang/rust#6023?
Ah. so, `y` being `i32` is kind of a bug, as you cannot really address any pixel < 0. If you have some time to work on this, can you please update function definition to read `pub fn set_pixel(&self, x: u32, y: u32, color: u16)` ? Then the addressing should be something along the lines of `(1u8 << (y % 8u32) as uint) as u8`