copied invert example into examples (#3069)

* simple invert example

* Update meson.build

examples/meson.build

@@ -2,6 +2,7 @@ examples = [
     'annotate-animated',
     'new-from-buffer',
     'progress-cancel',
+    'use-vips-func',
 ]
 
 foreach example : examples

examples/use-vips-func.c

@@ -0,0 +1,43 @@
+/* Example showing how to call a couple of vips functions using an input
+ * image file and creating an output file
+ * also gathers some info on the input image
+ */
+#include <stdio.h>
+#include <vips/vips.h>
+
+int
+main( int argc, char **argv )
+{
+  VipsImage *in;
+  double mean;
+  VipsImage *out;
+
+  if( VIPS_INIT( argv[0] ) )
+    vips_error_exit( NULL ); 
+
+  if( argc != 3 )
+    vips_error_exit( "usage: %s infile outfile", argv[0] ); 
+  
+  if( !(in = vips_image_new_from_file( argv[1], NULL )) )
+    vips_error_exit( NULL );
+
+  printf( "image width = %d\n", vips_image_get_width( in ) ); 
+
+  if( vips_avg( in, &mean, NULL ) )
+    vips_error_exit( NULL );
+
+  printf( "mean pixel value = %g\n", mean ); 
+
+  /* generate photo nexative - replace with other vips_ funcs */
+  if( vips_invert( in, &out, NULL ) )
+    vips_error_exit( NULL );
+
+  g_object_unref( in ); 
+
+  if( vips_image_write_to_file( out, argv[2], NULL ) )
+    vips_error_exit( NULL );
+
+  g_object_unref( out ); 
+
+  return( 0 );
+}