limit resize if shrink would drop axis <1px

vips_resize() will break the aspect ratio and limit resize on an axis if
it would result in an image of less than 1px on that axis

see https://github.com/lovell/sharp/issues/1782#issuecomment-508921306

ChangeLog

@@ -13,6 +13,8 @@ magic number [przemyslawpluta]
 - fix vipsthumbnail with pyr tiff [kleisauke]
 - text autofit could occasionally terminate early [levmorozov]
 - fewer warnings on tiffload [chregu]
+- vips_resize() breaks aspect ratio and limits shrink to prevent <1px 
+  dimensions [lovell]
 
 21/9/18 started 8.8.0
 - much faster smartcrop [lovell]

libvips/resample/resize.c

@@ -31,6 +31,8 @@
  * 3/12/18 [edwjusti]
  * 	- disable the centre sampling offset for nearest upscale, since the
  * 	  affine nearest interpolator is always centre 
+ * 7/7/19 [lovell]
+ * 	- don't let either axis drop below 1px
  */
 
 /*
@@ -213,6 +215,11 @@ vips_resize_build( VipsObject *object )
 		hscale *= int_hshrink;
 	}
 
+	/* Don't let either axis drop below 1 px.
+	 */
+	hscale = VIPS_MAX( hscale, 1.0 / in->Xsize );
+	vscale = VIPS_MAX( vscale, 1.0 / in->Ysize );
+
 	/* Any residual downsizing.
 	 */
 	if( vscale < 1.0 ) { 
@@ -424,6 +431,10 @@ vips_resize_init( VipsResize *resize )
  * @vscale, that factor is used for the vertical scale and @scale for the
  * horizontal.
  *
+ * If either axis would drop below 1px in size, the shrink in that dimension
+ * is limited. This breaks the image aspect ratio, but prevents errors due to
+ * fractional pixel sizes.
+ *
  * This operation does not change xres or yres. The image resolution needs to
  * be updated by the application. 
  *