Testing native image scaling

[Eli Zaretskii <eliz@gnu.org>]

Alan, could you please tell how you tested native image scaling with
the XRENDER extension, and perhaps show some Lisp or existing commands
you used for that?  E.g., did the features in thumbs.el work for you
in a build without Imagemagick?

I tried to implement this for MS-Windows, but I guess my understanding
of the internal workings of this is incomplete/incorrect, or my code
is buggy (or both), because I don't seem to be able to cause Emacs to
exercise the code when the original image's size and the size
requested by scaling differ.  For example, I thought that when scaling
is requested, x_set_image_size should be called and compute image
dimensions different from the original img->height and img->width, but
I seem to be unable to see this.  What am I missing?  Could you
perhaps describe the flow of calls when, e.g., the user types '+' on
an image in image-mode, and Emacs scales the image at point?

Thanks.

P.S. Incidentally, image-scaling-p is called in image.el also in
context where we want to know whether an image can be rotated by 90
degrees, and native scaling doesn't support that, does it?