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 12 minutes and 22 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #1594080
build

00:03:35

passed #1594085
build_android

00:03:15

passed #1594081
build_clang_libcpp

00:12:19

passed #1594084
build_clazy

00:02:54

passed #1594083
build_mingw64_fedora

00:03:24

passed #1594082
build_ubuntu_16_04

00:01:34