better test for output to target

We used to enable write to stdout if the first character of an output filename
was ".", eg.:

	vips copy x.jpg .png

But this will also enable write to stdout for things like:

	vips copy x.jpg ./y.png

This patch also tests that the rightmost "." in a filename is also the
first character.

Thanks barryspearce

See https://github.com/libvips/libvips/issues/1906

ChangeLog

@@ -13,6 +13,7 @@
 - check for overflow in gifload height [lovell]
 - fix msb_first default in ppm load and save [ewelot]
 - force binary mode on win for connection read / write [Alreiber]
+- better testing for output to target [barryspearce]
 
 6/9/20 started 8.10.2
 - update magicksave/load profile handling [kelilevi]

libvips/iofuncs/object.c

@@ -2176,6 +2176,17 @@ vips_object_print_arg( VipsObject *object, GParamSpec *pspec, VipsBuf *buf )
 	g_value_unset( &value );
 }
 
+/* Is a filename a target, ie. it is of the form ".jpg". Any trailing options 
+ * have already been stripped. Watch out for cases like "./x.jpg".
+ */
+static gboolean
+vips_filename_istarget( const char *filename )
+{
+	const char *p;
+
+	return( (p = strrchr( filename, '.' )) && p == filename );
+}
+
 /* Write a named arg to the string. If the arg does not need a string (see
  * above), arg will be NULL.
  */
@@ -2209,7 +2220,7 @@ vips_object_get_argument_to_string( VipsObject *object,
 
 		vips__filename_split8( arg, filename, option_string );
 
-		if( vips_isprefix( ".", filename ) ) {
+		if( vips_filename_istarget( filename ) ) {
 			VipsTarget *target;
 
 			if( !(target = vips_target_new_to_descriptor( 1 )) )