Xft: Change xftfont_min_descent to return max descent

Like the font's ascent, its descent is also always positive.  That means
for the common use-case of getting the greatest descent for a list of
fonts, one can simply return the maximum of all descents.  In this way,
it is possible to center text that's using multiple fonts by simply
calculating the maximum ascent and descent.  This is also done in
xmobar's MinXft library[1], which this part of the code is modeled
after.

Fixes: 0cc513a
Related: xmonad/xmonad-contrib#646
[1]: https://github.com/jaor/xmobar/blob/master/src/Xmobar/X11/MinXft.hsc#L94

Author: slotThe

CHANGES.md

@@ -11,7 +11,7 @@
     possibly uses different fonts for different parts of the string and
     returns the accumulative extents.
 
-  * Added the functions `xftfont_max_ascent`, `xftfont_min_descent`, and
+  * Added the functions `xftfont_max_ascent`, `xftfont_max_descent`, and
     `xftfont_max_height` to gain information about a non-empty list of
     `XftFont`s.
 

Graphics/X11/Xft.hsc

@@ -28,7 +28,7 @@ module Graphics.X11.Xft ( XftColor
                         , xftfont_ascent
                         , xftfont_max_ascent
                         , xftfont_descent
-                        , xftfont_min_descent
+                        , xftfont_max_descent
                         , xftfont_height
                         , xftfont_max_height
                         , xftfont_max_advance_width
@@ -197,8 +197,8 @@ foreign import ccall "XftFontClose"
 xftfont_max_ascent :: NonEmpty XftFont -> IO Int
 xftfont_max_ascent = fmap maximum . mapM xftfont_ascent
 
-xftfont_min_descent :: NonEmpty XftFont -> IO Int
-xftfont_min_descent = fmap minimum . mapM xftfont_descent
+xftfont_max_descent :: NonEmpty XftFont -> IO Int
+xftfont_max_descent = fmap maximum . mapM xftfont_descent
 
 xftfont_max_height :: NonEmpty XftFont -> IO Int
 xftfont_max_height = fmap maximum . mapM xftfont_height