Currently, gradients are drawn by filling a series of quadrilaterals,
with one side equal to a gradient line and the other side equal to the
edge of the bounding rectangle of the area being filled. A single
pixel can be filled many times, and that causes flicker.

Theoretically, a line gradient covers the area enclosed by the
starting and ending gradient lines, which intersect the start and end
points respectively and are perpendicular to a line joining the start
and end points.

Negative lfHeight values do not include internal leading. This was
making fonts created from LOGFONT structures larger than they should be.

user32/tests: Show that vertical and horizontal window scroll bar info is not created independently.