cairo-util: fix shadows for small clients

If the client is small (< 128 pixels in any ward),
then the shadows overlap and create dark lines behind clients.
This is a problem mosly with pop-up menues. The lines become observable
when the menu has less than three items. The other case is when
the client doesn't restrict its size when resizing (try
'weston-eventdemo --max-width=1 --max-height=1' for example)

This fixes a part of the bug:
https://bugs.freedesktop.org/show_bug.cgi?id=78511



v2:
  - rework computing of the size of corners
  - rewrite some comments
  - rename tile_mask to render_shadow (in separate patch)

Signed-off-by: Marek Chalupa <mchqwerty@gmail.com>
Tested-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>