Handle unsupported compositors more gracefully

If the virtual keyboard or virtual pointer protocols are not supported,
produce a meaningful error message rather than dereference NULL and
segfault. Related to #16

src/wayland.c

@@ -347,6 +347,15 @@ bool wlSetup(struct wlContext *ctx, int width, int height)
 	wl_display_dispatch(ctx->display);
 	wl_display_roundtrip(ctx->display);
 
+	if (!ctx->pointer_manager) {
+		logErr("Compositor does not support wlr_virtual_pointer_unstable_v1");
+		return false;
+	}
+	if (!ctx->keyboard_manager) {
+		logErr("Compositor does not support virtual_keyboard_unstable_v1");
+		return false;
+	}
+
 	ctx->pointer = zwlr_virtual_pointer_manager_v1_create_virtual_pointer(ctx->pointer_manager, ctx->seat);
 	ctx->keyboard = zwp_virtual_keyboard_manager_v1_create_virtual_keyboard(ctx->keyboard_manager, ctx->seat);
 	if(wlKeySetConfigLayout(ctx)) {