Debugger: SourceView: Fix several drawing bugs.

- MarkerView: If the view was resized, and the newly revealed region was
after the last source line, it wouldn't be redrawn properly.
- TextView: Reset low color for painting the region after the
source.  Otherwise, if either the instruction pointer or a breakpoint
were at the last line of the current source, the empty space after would
be filled in that color rather than the background color.