pngsave interlaced makes a copy of the image

interlaced png is hard to write: you have to make seven passes over the
image to make the whole file. It advertises itself as sequential, it
needs to only ake a single pass. Therefore, it must take a copy in
memory of the image to write.

ChangeLog

@@ -4,6 +4,7 @@
 - don't exit() on memleak detected, just warn
 - add "autocrop" option to openslide load
 - argh fix affine, again
+- pngsave in interlaced mode make a copy of the whole image
 
 4/7/14 started 7.40.4
 - fix vips_rawsave_fd(), thanks aferrero2707 

TODO

@@ -2,9 +2,6 @@
 
   change caching:
 
-  - png and jpg interlaced write need to make a memory copy before writing,
-    they advertse as seq, they need to be seq
-
   - autorotate should make a mem copy if it find rotation is necessary
 
   - remove second vips_cache()

libvips/foreign/vips2jpeg.c

@@ -58,6 +58,7 @@
  * 	- attach IPCT data (app13), thanks Gary
  * 2/10/13 Lovell Fuller
  * 	- add optimize_coding parameter
+ * 	- add progressive mode
  * 12/11/13
  * 	- add "strip" option to remove all metadata
  * 13/11/13

libvips/foreign/vipspng.c

@@ -777,10 +777,18 @@ write_vips( Write *write, int compress, int interlace )
 	if( setjmp( png_jmpbuf( write->pPng ) ) ) 
 		return( -1 );
 
-	/* Check input image.
+	/* Check input image. If we are writing interlaced, we need to make 7
+	 * passes over the image. We advertise ourselves as seq, so to ensure
+	 * we only suck once from upstream, switch to WIO. 
 	 */
-	if( vips_image_pio_input( in ) )
-		return( -1 );
+	if( interlace ) {
+		if( vips_image_wio_input( in ) )
+			return( -1 );
+	}
+	else {
+		if( vips_image_pio_input( in ) )
+			return( -1 );
+	}
 	if( compress < 0 || compress > 9 ) {
 		vips_error( "vips2png", 
 			"%s", _( "compress should be in [0,9]" ) );
@@ -813,7 +821,7 @@ write_vips( Write *write, int compress, int interlace )
 		in->Xsize, in->Ysize, bit_depth, color_type, interlace_type, 
 		PNG_COMPRESSION_TYPE_DEFAULT, PNG_FILTER_TYPE_DEFAULT );
 
-	/* Set resolution. libpnbg uses pixels per meter.
+	/* Set resolution. libpng uses pixels per meter.
 	 */
 	png_set_pHYs( write->pPng, write->pInfo, 
 		VIPS_RINT( in->Xres * 1000 ), VIPS_RINT( in->Yres * 1000 ),