When a clamped gradient has multiple color stops at 0.0, the last one
will fill GradientData[0]. When a clamped gradient has multiple color
stops at 1.0, the first one will fill GradientData[N-1]. Both of these
conditions will cause the area outside of the gradient to not be filled
with the correct clamped color, which is the first or last gradient
color stop.

Filling the first or last color entry of GradientData with the first or
last color stop isn't exactly correct either because the first or last
color stop entry is only needed in this case for the range outside of
[0, 1].

This commit changes the GradientData so that the first color entry is
used for offset < 0, the last color entry is used for offset >= 1, and
the color entries in between are for the range in between. The first and
last color entries are then filled with the correct colors for those
ranges.