Fix brush rounding.

The use of floor appeared to cause minor inconsistencies in the position
of brush edges for different positions along a scale.  I believe floor
was originally used to ensure the brush would display more crisply.

Using round would be an alternative fix.  However, removing any kind of
rounding in pixel-space ensures that transitioning an extent (set in
data-space) will finish with the correct extent, determined by inverting
the pixel-space extent.

Of course, there may be a small precision error introduced by inverting
the pixel-space extent, which we may also want to address anyway, in
which case I suppose we could still use round.