Add option imagemagick-types-enable

* lisp/image.el: For clarity, call imagemagick-register-types at
top-level, rather than relying on a custom :initialize.
(imagemagick-types-enable): New option. 
(imagemagick-register-types): Respect imagemagick-types-inhibit.
If disabling support, remove elements altogether rather
than using an impossible regexp.
(imagemagick-types-inhibit): Give it the default init function.

* src/image.c (Fimagemagick_types): Doc fix.

* etc/NEWS: Mention this.

Fixes: debbugs:11557

1 files changed, 5 insertions, 1 deletions

diff --git a/etc/NEWS b/etc/NEWS
index dd22abfb9d1..ff662a70c1c 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -60,12 +60,16 @@ name, group names known to the system (where possible).
 ** ImageMagick support, if available, is automatically enabled.
 It is no longer necessary to call `imagemagick-register-types'
 explicitly to install ImageMagick image types; that function is called
-automatically at startup or when customizing `imagemagick-types-inhibit'.
+automatically at startup, or when customizing imagemagick-types-enable
+or imagemagick-types-inhibit.
 
 *** Setting `imagemagick-types-inhibit' to t now disables the use of
 ImageMagick to view images.  You must call imagemagick-register-types
 afterwards if you do not use customize to change this.
 
+*** The new variable `imagemagick-types-enable' also affects which
+ImageMagick types are treated as images.
+
 ** String values for `initial-buffer-choice' also apply to emacsclient
 frames, if emacsclient is only told to open a new frame without
 specifying any file to visit or expression to evaluate.