colors were a mixture of 4bpp integers (0=white, 15=black) and
fractional blackness levels (0=white, 1.0=black) before. This is
now unified to use the color specification of the Blitbuffer API.
The #1011 bug is caused by the `contains` check of link box with
visible area. When the link is at the very right (left?) of
the page, enlarged link box often exceeds the page bound rendering
the mentioned `contains` check failed. This patch uses an `intersectWith`
test to check if the link is in current page.