jank-imgui demo

Demonstrates calling the C++ imgui library from jank to render a simple interactive GUI.

1. Compile imgui as a shared library:

   make libimgui.so
   

2. Run the jank demo (include/search paths may differ):

   jank -L/usr/lib/x86_64-linux-gnu -lglfw -lGL \
       -L. -limgui -Iimgui -Iimgui/backends \
       --module-path=. run-main imgui-demo