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This changeset also rearranges native image transform code
for other platforms to make it cleaner, and also removes
the support for native cropping.  For the discussions, see
https://lists.gnu.org/archive/html/emacs-devel/2019-06/msg00242.html

* src/w32term.c (w32_image_rotations_p, transform): New functions.
(w32_draw_image_foreground): If image rotation is requested
and supported, call PlgBlt to transform the image.
(w32_initialize): Populate the PlgBlt function pointer if it
is supported.
* src/w32term.h (w32_image_rotations_p): Add prototype.
* src/dispextern.h (struct image) [HAVE_NTGUI]: New member xform.
* src/image.c (compute_image_rotation): Renamed from
image_set_rotation.  Only compute and returns the rotation
angle; leave the matrix calculation for later.  Log an error
message if the :rotation parameter is not a number.
(image_set_crop): Function deleted.  We no longer support
native cropping, as one can display an image slice instead.
(image_set_transform): Compute the transform matrix in its
entirety here, in two variants: one for XRender and Cairo, the
other for NS and MS-Windows.  call compute_image_size and
compute_image_rotation internally.
(lookup_image) [HAVE_NATIVE_TRANSFORMS]: Call only
image_set_transform.  No need to pass the transform matrix to
image_set_transform.
(Fimage_transforms_p): Return a list of transform capabilities
rather than a simple boolean.  Support TTY frames as well.
* src/nsimage.m (setTransform:): Don't invert the matrix, as
it is already inverted in image.c.

* test/manual/image-transforms-tests.el (test-cropping): State
in the text that only ImageMagick supports cropping.

* doc/lispref/display.texi (Image Descriptors): Update the
documentation of native image transforms.
(ImageMagick Images): Move the description of ':crop' here.

* etc/NEWS: Minor copyedits of the feature announcement.