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 56 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #1278052
build

00:03:15

passed #1278057
build_android

00:00:59

passed #1278053
build_clang_libcpp

00:03:28

passed #1278056
build_clazy

00:02:18

passed #1278055
build_mingw64_fedora

00:04:55

passed #1278054
build_ubuntu_16_04

00:01:42