xwfont_library needs to utilize the OpenGL loader for shared

[xeekworx]

An OpenGL loader exists in the project as xwfglad; basically using dav1d's glad with the prefixes changed so that they don't collide with any library user's glad (if they use it).

If the xwfont_library is built as a shared library (intended for use in C# tools in the future) then the included glad loader needs to actually be utilized so the OpenGL functions used by the NanoVG or future OpenGL rendering backends aren't bad pointers.

It currently works for static builds, because it relies on the library user to load enough of OpenGL for the library to work. It is technically their responsibility. I've not designed the xwfont_library to work outside of static builds unless it's for font generation in future tools that I haven't built yet.