TextSelectionPainter: support glyphless fonts

in text selections, by:

 - Ignoring to draw characters with it.
 - Painting the selection's background as transparent.

Fixes issue #157

Based on inital work by Nelson Benitez, but changed to be not
tesseract specific.
6 jobs for 157-glyphless-font in 4 minutes and 42 seconds
latest
Status Job ID Name Coverage
  Build
passed #317718
build

00:03:25

passed #317723
build_android

00:01:38

passed #317719
build_clang_libcpp

00:03:25

passed #317722
build_clazy

00:04:41

passed #317721
build_mingw64_fedora

00:02:53

passed #317720
build_ubuntu_16_04

00:01:39