Pixel write with drawoptions

[l4l]

I'd like to use rusttype for font-rendering instead of font-kit. Its API provides a set of points for the specified font render, for now I made it work via dt.fill_rect(x, y, 1., 1., color, &opts), but I guess there is a better way of achieving this. I don't want to manually edit the pixel buffer, because there will be no anti-aliasing.

[jrmuizel]

A possible better way to do this is with DrawTarget::mask(). i.e. you construct a mask with the glyph like https://github.com/redox-os/rusttype/blob/057f2d1d521667a90d082a34913ed7743754bf02/src/gpu_cache.rs#L972 does and then use DrawTarget::mask() to draw it at the correct location.

That being said, this is still going to be awkward. Overall, the text rendering in raqote needs a rework. My grand plan is to move the font rendering code in https://github.com/servo/webrender/tree/master/webrender/src/platform to a separate crate and use that for caching a drawing glyphs. As part of this, it should be possible to add support for a pure rust text backend as well.