Fix text highlighting in UIScrollView

Highlight target layers with non-zero bounds.origin exhibited broken
text highlighting behavior. Highlights would appear offset by the
layer’s bounds.origin, and would not align with their target text. This
most often appeared in UIScrollView, where highlights would appear
incorrectly offset once the scroll view was scrolled. Fix by accounting
for bounds.origin in the calculations of the text overlay highlight
rects.
Merge pull request #84 from bcunning/highlight-origin-fix

Fix text highlighting in UIScrollView