Hello,

The attached patch examines the actual glyph's outline with DirectWrite 
to compute exact values for ascent and descent and should solve the 
problem for any font. Specifically, Sans Serif Collection works well 
now, and everything on HELLO.txt looks good on my system.

This adds some overhead, but I didn't find it noticeable.  In case 
someone does, we could cache measurements as the GDI backend does.

When computing the bounding box for bezier curves, I just used minmax 
values for start/end and control points, instead of actually evaluating 
the curve.  This reduces the overhead and loses some precision, but I 
didn't see any real difference.