fix paintbox speed

[jcupitt]

When drawing wide lines, draw_line was calling vips_image_inplace() and vips_image_wio_input() for each pixel. This triggered invalidate, and when the operation cache was full of cached getpoint() calls from the info bar, made drawing very slow.

This might fix the crashes with paint + infobar enabled too.

[jcupitt]

@kleisauke this might fix the infobar crashes too, I'm not certain.

[jcupitt]

Ooop, nope, it just crashed again.

It's a lot quicker though, so that's nice.

[kleisauke]

Nice! It does seem much faster, although I was also able to provoke a crash. :(

Looking at the doc comment for vips_image_inplace():
https://www.libvips.org/API/current/method.Image.inplace.html

How about adding the same check we added to vips_image_set() and vips_image_remove() via commit libvips/libvips@80037fc?

--- a/libvips/iofuncs/image.c
+++ b/libvips/iofuncs/image.c
@@ -3600,6 +3600,14 @@ vips__image_wio_output(VipsImage *image)
 int
 vips_image_inplace(VipsImage *image)
 {
+#ifdef DEBUG_LEAK
+	/* Warn if inplace is being called on a shared image.
+	 */
+	if (vips__leak &&
+		G_OBJECT(image)->ref_count > 2)
+		printf("vips_image_inplace: shared image %p\n", image);
+#endif /*DEBUG_LEAK*/
+
 	/* Do an vips_image_wio_input(). This will rewind, generate, etc.
 	 */
 	if (vips_image_wio_input(image))

I noticed that this warning is triggered when painting, but I'm not sure whether it's a red herring.