Handle compact arguments in arc commands.

This is used by recent versions of the Adwaita icons:
https://people.gnome.org/~federico/blog/do-not-use-librsvg-2.40.x.html

--- librsvg-2.40.21/rsvg-path.c	2021-11-27 22:32:58.621881500 +0000
+++ librsvg-2.40.21/rsvg-path.c	2021-11-27 22:32:49.714942405 +0000
@@ -640,6 +640,11 @@
                 } else if (c >= '0' && c <= '9') {
                     value = c - '0';
                     in = RSVGN_IN_INTEGER;
+                    /* Flag arguments can be one digit without trailing space. */
+                    if (ctx->cmd == 'a' && (ctx->param == 3 || ctx->param == 4)) {
+                        end = TRUE;
+                        length++;
+                    }
                 }
                 break;
             case RSVGN_IN_INTEGER: /* Previous character(s) was/were digit(s) */