[plt-scheme] dc<%> scale and text

A dc<%> object supports independent vertical and horizontal scales for
drawing. If the scales are different, then drawn objects are squashed
or stretched according to the two scales. Up to now, however, in a
canvas or bitmap dc<%>, text grew or shrank only according to one of
the scaling factors --- usually the vertical scale, but it wasn't well
defined.

With changes that are now exp-tagged in CVS, text squashes and
stretches like all other drawing operations, except under Windows
95/98/Me.

Due to the way that fonts work on most platforms (each character
ultimately has a certain size in pixels), scaling by a factor of X
doesn't make a character or string of text scale by exactly X, but it's
normally close. When the horizontal and vertical scales are the same,
the actual scaling of text is now closer to the requested scaling than
it used to be.

Matthew