Add unit test for Annotation::renderToImage

This test checks the expected ratio of pixels in known color inside the
rendered image, with minor tolerance, in order to improve test
maintainability.

Note: The test covers ArthurOutputDev, which requires a QGuiApplication
due to internal usage of QRawFont. Therefore we have a dependency
on running X during test execution.
4 jobs from render-single-annotation in 11 minutes and 40 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #36822
build

00:09:54

passed #36823
build_clang_libcpp

00:09:49

passed #36824
build_gcc_49

00:11:39

passed #36825
build_mingw64_fedora

00:10:00