Conservatively estimate geometry bounds

bug:10761696

Avoids a case where a rect with top coordinate of (e.g.) 0.51f is
assumed to not draw in the first row of pixels, which leads to it not
being clipped. Since rounding can cause it to render in this first
pixel anyway, we very slightly expand geometry bounds.

Now, in ambiguous cases, the geometry bounds are expanded so clipping
is more likely to happen.

Change-Id: I119b7c7720de07bac1634549724ffb63935567fc