Slice images based on their height in SHR, not their zoom level

Slice images based on their height in SHR, not their zoom level

* lisp/net/shr.el (shr-sliced-image-height): New option...
(shr-put-image): ... use it.  Compute the number of slices in relation
to the image height; this way, each slice is roughly the height of a
line of ordinary text.

* test/lisp/net/shr-tests.el (shr-test/zoom-image): Update test, since
zooming no longer necessarily triggers slicing.

* doc/misc/eww.texi (Advanced): Document 'shr-sliced-image-proportion'.

* etc/NEWS: Announce this change.

(cherry picked from commit 3ce7e4ee3f1f8bf85c2c455ac624bec6c7cd10a8)