Previously we transformed each rasterized point when drawing a line.
Now we transform the lines' endpoints instead.
That means running two transforms per line instead of transforms for
each pixel. It is not clear that the overhead for the fast path is
still worth it. If we still want to optimize identity and translations,
it is probably better to do that inside AffineTransform.
In addition this will behave nicer if the transform includes scaling.
Previously this would rasterize lines before scaling. Which means
drawing too many points when scaling down, and not drawing enough
points when scaling up.
With the new approach we will automatically rasterize at pixel scale.
This is essentially the same as OpenGL, where vertices are transformed
and rasterization happens in screen space.
Instead of taking a callback that performs the coordinate transformation
we now just take a bool template parameter.
Thanks to Idan for suggesting this! :^)
If the effective 2D transform is just a basic translation, we now simply
translate the underlying painter before & after drawing AA lines.
This avoids all the extra math that otherwise has to happen when mapping
points through an affine transform.
This noticeably increase "mousing around" performance on Wikipedia. :^)
