SpaceDrawer: Draw tabs so that their endpoints align
SpaceDrawer: Draw tabs so that their endpoints align

Previously, tabs were drawn with padding of 1/8 of the width of the
actual tab character.  However, because tabs aren't always the same
width (even in the same document), this could cause their endpoints to
not line up.  This resolves this issue by making the padding on either
side of the symbol instead proportional to the height of the tab, which
is fixed for a given font size.

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