Logo (Image): adds animation (gif / apng) support

doc/help.json

@@ -293,6 +293,7 @@
         {
             "long": "logo-cache",
             "desc": "Specify how the image logo cache is used",
+            "remark": "See \"--help logo-animation-frame\" for the one case where a cached rendering can outlive the image type that produced it",
             "arg": {
                 "type": "enum",
                 "optional": true,
@@ -304,6 +305,21 @@
                 }
             }
         },
+        {
+            "long": "logo-animation-frame",
+            "desc": "Select which frame of an animated image logo to render",
+            "remark": [
+                "Animated kitty graphics only, and only if the image source is animated; 1 or higher renders that frame as a still image, 0 plays the animation",
+                "A selected frame is cached exactly like the first frame is, sharing its entries; the frame number is recorded next to them, so changing this option re-renders",
+                "That record is per cache directory, not per image type: switching --logo-type between two types that both have a cached rendering here, and asking for a different frame, can pair one type's rendering with the other's frame number",
+                "Run \"--logo-cache regen\" after such a switch"
+            ],
+            "arg": {
+                "type": "num",
+                "optional": true,
+                "default": 1
+            }
+        },
         {
             "long": "file",
             "desc": "Short for --logo-type file --logo <path>",