text autofit could sometimes underfit

The autofit loop would terminate if either width or height fitted exactly,
but this could happen very early by chance. This patch makes it keep
looping until it finds a dpi which just fits.

See https://github.com/libvips/libvips/issues/1352

ChangeLog

@@ -11,6 +11,7 @@ magic number [przemyslawpluta]
 - remove 256 band limit in arithmetic.c [erdmann]
 - disable Orc if building with CET [lovell]
 - fix vipsthumbnail with pyr tiff [kleisauke]
+- text autofit could occasionally terminate early [levmorozov]
 
 21/9/18 started 8.8.0
 - much faster smartcrop [lovell]

libvips/create/text.c

@@ -28,6 +28,8 @@
  * 16/3/19
  * 	- add `justify`
  * 	- set Xoffset/Yoffset to ink left/top
+ * 27/6/19
+ * 	- fitting could occasionally terminate early [levmorozov]
  */
 
 /*
@@ -58,8 +60,8 @@
  */
 
 /*
-#define DEBUG
  */
+#define DEBUG
 
 #ifdef HAVE_CONFIG_H
 #include <config.h>
@@ -204,18 +206,13 @@ vips_text_get_extents( VipsText *text, VipsRect *extents )
 	return( 0 );
 }
 
-/* Return -ve for extents too small, 0 for a fit, +ve for extents too large.
+/* Return -ve for extents too small, +ve for extents too large.
  */
 static int
 vips_text_rect_difference( VipsRect *target, VipsRect *extents )
 {
-	if( vips_rect_includesrect( target, extents ) ) {
-		if( target->width == extents->width ||
-			target->height == extents->height )
-			return( 0 );
-
+	if( vips_rect_includesrect( target, extents ) ) 
 		return( -1 );
-	}
 	else
 		return( 1 );
 }
@@ -261,10 +258,9 @@ vips_text_autofit( VipsText *text )
 			previous_difference = difference;
 		}
 
-		/* Hit the size, or we straddle the target.
+		/* Stop if we straddle the target.
 		 */
-		if( difference == 0 ||
-			difference != previous_difference ) 
+		if( difference != previous_difference ) 
 			break;
 
 		previous_difference = difference;