Fix valgrind warnings triggered in vertical mode.

Fix valgrind warnings triggered in vertical mode.

Now that Chrome is rendering subpixel text, I was able to try running
the renderer process under valgrind, which turned up a number of
issues.

First, I was calculating the stride of vertical LCD glyphs wrong
(typo).

Secondly, I was going horribly wrong when a glyph was being blitted at
the edge of a bitmap. I suspected something was wrong with the code,
but I wasn't clear enough with the structure of the code when writing
it to figure out what the correct solution was.

[reed, 2009-07-22 20:50:16]

LGTM