Default lineHeight based on the font size.

Flot 0.8.0 used the default font size, typically derived from the
placeholder, as the basis for the default lineHeight.  This produced
incorrect results when a font.size was provided explicitly, and it
differed from the placeholder’s CSS size.

Fixed by waiting to default lineHeight until the actual font size has
been resolved.  Fixes #1131.
Merge pull request #1199 from dnschnur/line-height-default-fix

Default lineHeight based on the font size.