Center characters vertically

Currently math challenges look a bit weird because the math operators
are smaller than the numbers and everything is aligned at the top of the
image.

This change moves all characters to the vertical center, making math
challenges a bit nicer to look at.

captcha/views.py

@@ -94,13 +94,14 @@ def captcha_image(request, key, scale=1):
         charimage = charimage.crop(charimage.getbbox())
         maskimage = Image.new("L", size)
 
+        yoffset = (size[1] - charimage.size[1]) // 2
         maskimage.paste(
             charimage,
             (
                 xpos,
-                DISTANCE_FROM_TOP,
+                yoffset,
                 xpos + charimage.size[0],
-                DISTANCE_FROM_TOP + charimage.size[1],
+                charimage.size[1] + yoffset,
             ),
         )
         size = maskimage.size