Fall back to ANIMATION_NONE if startup-animation is zoom

get_animation_type() parses "none", "zoom" and "fade" but for the
startup animation, we only support "none" and "fade".  If we get "zoom"
just fall back to "none" like we do for all unrecognized strings.

https://bugs.freedesktop.org/show_bug.cgi?id=71006

src/shell.c

@@ -490,6 +490,9 @@ shell_configuration(struct desktop_shell *shell)
 					 "startup-animation", &s, "fade");
 	shell->startup_animation_type = get_animation_type(s);
 	free(s);
+	if (shell->startup_animation_type == ANIMATION_ZOOM)
+		shell->startup_animation_type = ANIMATION_NONE;
+
 	weston_config_section_get_uint(section, "num-workspaces",
 				       &shell->workspaces.num,
 				       DEFAULT_NUM_WORKSPACES);