Because code comments are rendered using italics, and because italics
can sometimes draw text outside of their bounding boxes, some characters
may be visually obscured when they are adjacent to an annotation with a
background color -- particularly intraline difference annotations. With
this change, intraline difference background is given an alpha channel
so that such characters are not obscured.
Bug: Issue 4785
Change-Id: I5372e6f4def5e69126288965eb9829e5680c2102
3894bb9f2a