Correct stride for drawing to cached glyph bitmap

fixes the glyph cachebuffer index to account for pixelbuffer
format size.

issue: in launcher home screen create icon group and rename the
group to some text string with emo-icons (smilies). without this
change the drop shadows for emo-icons will look bad.

Change-Id: I525a9219d35f4541363fd64e7fc5fd1053269725