Implement the ImagePixelGetter trait

[rfuest]

PR #34 did add a pixel getter method to Bmp and RawBmp, but didn't yet implement the e-g ImagePixelGetter trait. The Bmp::pixel method should be moved to a ImagePixelGetter trait impl when a new e-g version with this trait is released.

[Nerixyz]

I'd like to add this. Though I think it should be named something like GetPixel which is more Rust-like (when comparing to something like FromStr, AddAssign, IntoFuture).

[rfuest]

The trait already exists in the embedded-graphics master version, but not in a released version. GetPixel might be a better name. If you would like to change it please open an issue or PR in the embedded-graphics repo.

[Nerixyz]

Oh my bad, I didn't see that on the docs since it's not released yet. But I suppose it's a bit too late to change the name now. I was just thinking if the trait is new then one could still decide on the name.

[rfuest]

IMO it's not too late to change the name until there has been a release on crates.io.