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Prefer librsvg for display of splash.svg
When both librsvg and Imagemagick are available, Emacs should
prefer librsvg to render SVG images.  However, Emacs was using
Imagemagick to render its own splash.svg file because
image-type-from-file-header returned nil for that file.
* lisp/image.el (image-type-from-buffer)
(image-type-from-file-header): Look at the first 8192 bytes of
the image, not just the first 256.  For Emacs’s own splash.svg
file, image-type-header-regexps needs to look at 939 bytes.
8192 bytes is a reasonable number nowadays given typical file
system design.
* test/lisp/image-tests.el (image-tests--emacs-images-directory):
New contant.
(image-type-from-file-header-test): New test.