fix a couple of clang warnings

libvips/conversion/composite.cpp

@@ -1721,7 +1721,18 @@ vips_composite2( VipsImage *base, VipsImage *overlay, VipsImage **out,
 	va_list ap;
 	int result;
 
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wvarargs"
+
+	/* Triggers a clang compiler warning because mode might not be an int.
+	 * I think the warning is harmless for all platforms we care about.
+	 */
 	va_start( ap, mode );
+
+	g_assert( sizeof( mode ) == sizeof( int ) );
+
+#pragma clang diagnostic pop
+
 	result = vips_call_split( "composite2", ap, base, overlay, out, mode );
 	va_end( ap );
 

libvips/foreign/heifsave.c

@@ -300,7 +300,8 @@ vips_foreign_save_heif_build( VipsObject *object )
 		return( -1 );
 
 	error = heif_context_get_encoder_for_format( heif->ctx, 
-		heif->compression, &heif->encoder );
+		(enum heif_compression_format) heif->compression, 
+		&heif->encoder );
 	if( error.code ) {
 		if( error.code == heif_error_Unsupported_filetype ) 
 			vips_error( "heifsave", 

libvips/include/vips/foreign.h

@@ -639,6 +639,8 @@ int vips_dzsave( VipsImage *in, const char *name, ... )
  * @VIPS_FOREIGN_HEIF_COMPRESSION_AV1: aom
  *
  * The compression format to use inside a HEIF container. 
+ *
+ * This is assumed to use the same numbering as %heif_compression_format.
  */
 typedef enum {
 	VIPS_FOREIGN_HEIF_COMPRESSION_HEVC = 1,