Calculates the snip's width, height, descent (amount of height which should be drawn below the baseline), space (amount of height which is ``filler'' space at the top), and horizontal spaces (amount of width which is ``filler'' space at the left and right).
A drawing context is provided for the purpose of finding font sizes, etc., but no drawing should occur. The snip's left and top locations are provided in buffer corrdinates. In an editor bufferm, the y-coordinate is the line's top location; the snip's actual top location is potentially undetermined until its height is known.
Fills in all boxes with 0.0.