Rather than declaring a big C++ literal for these strings, they are now
proper `.vert` and `.frag` files that will become embedded into the
application at build-time. It also allows for clang-format to format the
glsl files the same as our C++ code. CMake will also automatically track
and re-embed the file if it detects that the glsl source files have
changed since the last build. Ex, making a change to
`opengl_display.frag` and compiling will automatically recompile and
link the resource-target that it is associated with.
