raise the PNG text chunk limit

Raise the PNG text chunk limit to 50, since it seems some real images
have a text chunk per line of exif data.

See https://github.com/libvips/libvips/issues/2433

libvips/foreign/pforeign.h

@@ -45,7 +45,9 @@ extern "C" {
 )
 #endif /*HAVE_CHECKED_MUL*/
 
-#define MAX_PNG_TEXT_CHUNKS 20
+/* We've seen real images with 28 chunks, so set 50.
+ */
+#define MAX_PNG_TEXT_CHUNKS 50
 
 void vips__tiff_init( void );
 

libvips/foreign/vipspng.c

@@ -570,7 +570,8 @@ png2vips_header( Read *read, VipsImage *out )
 		if( !read->unlimited && 
 			num_text > MAX_PNG_TEXT_CHUNKS ) {
 			vips_error( "vipspng", 
-				"%s", _( "too many text chunks" ) );
+				_( "%d text chunks, image blocked" ),
+			       num_text );
 			return( -1 );
 		}