1. Using container_cairo_pango for running tests.
2. Added Dockerfile to create docker container for building litehtml
tests
3. Added script run_render_tests.sh to build litehtml tests in the
docker container
4. Added render2png target (cmake -DLITEHTML_BUILD_RENDER2PNG). This
utility allow to render html file into a png file.
5. Tests building are off by default.
Use "cmake -DLITEHTML_BUILD_TESTING=ON" command to buid tests
6. Added container_cairo_pango::get_font_options. Overwrite this method
to control fonts rendering.
