[RFC] Invert -Wconversion handling & fix some warnings.

Instead of selectively enabling `-Wconversion`, I think it might be better to instead disable this for some files to avoid accidentally missing some (e.g. newly created) files. This will also give a list of "problematic" files right in the CMake file.

Also added two commits fixing the warnings in `cursor_shape.c` and `digraph.c`.