Allow changing the default fonts using compiler flags

[wijnen]

For Debian, we want to use the packaged font instead of the bundled one. I made a patch to allow this change using compile flags. Please consider including it.

font.txt

[MBilderbeek]

Hmm, not really great that we define the default font both in _osd_menu.tcl (although it's hardly used anymore) and in the C++ code (but only in the OSD part, which is not used very intensively anymore). And then it's also kind of defined in the ImGUI font setting definition...