Simplify the drawing of buffered highlight rectangles
Review Request #130214 - Created Aug. 3, 2017 and submitted
Previously, they were drawn by modifying individual pixels of the QImage. With this patch we simply use a QPainter. That makes the code a lot shorter.
This follows a suggestion from https://blog.hetzenecker.me/2017-06-30-gsoc-kdegraphics-okular/
The code that is removed by this patch is the sole consumer of the has_alpha variable. Consequently the compiler now warns about an unused variable. Should I send another patch removing that variable?
Tested with pdf and odt documents, and different text colors, and I could not find any functional difference. In particular, testing with pdf and odt makes sure that both branches of the if(has_alpha) conditional are tested.