Implement proof output
Improve coding style of proof mode.
Proof mode: stroke and fill texts to improve visibility.
Merge pull request #371 from duanyao/proof_output

Implement proof output
Implement covered text handling.
Handle texts covered by paths; concentrate drawing tracing codes to DrawingTracer calss; disable CSS drawing.
Handle clips during processing of covered text.
Code cleaning
Make SplashBackgroundRenderer working with --process-covered-text
Delete an unintentionally commited file.
Add check of nullptr to fallback_bg_renderer; remove outdated comment.
Fix covered text processing issue (in DrawingTracer::set_ctm) when zoom != 1
Fix Travis build
Improve DrawingTracer::stroke() by break path into steps.
Fix issue in char displacement calculation.
Improve stroke handling in DrawingTracer by simulating line join.
Fix of "Fix issue in char displacement calculation".
Fix word space problem in HTMLRenderer::drawString().
Allow covered text handling (--process-covered-text) to work without cairo.
Improve covered text handling: 1. take care of chars corespond to 0 or more than one unicode points;
2. merge sibling invisiable spans; 3. improve interfaces of HTMLLineState and HTMLRenderer;