Most of the calls to rsvg_g_warning() indicate programming errors,
mostly around functions being called at the wrong time (e.g. the
is_loaded() checks).  Those are panics now.

For the other genuine warnings, we cannot call rsvg_g_warning_from_c()
when building librsvg_crate, since that does not link to the C part of
librsvg.so.  However, for the cases where that is used, those cases
do return meaningful Rust error codes, so they don't need warnings.

So, the rsvg_internals crate now has a feature "c-library" - when
enabled, it will call rsvg_g_warning_from_c(); when disabled, it will
do nothing.