osc: kitty notifications: updated support for icons

This implements the suggested protocol discussed in
https://github.com/kovidgoyal/kitty/issues/7657.

Icons are handled by loading a cache. Both in-band PNG data, and
symbolic names are allowed.

Applications use a graphical ID to reference the icon both when
loading the cache, and when showing a notification.

* 'g' is the graphical ID
* 'n' is optional, and assigns a symbolic name to the icon
* 'p=icon' - the payload is icon PNG data. It needs to be base64
  encoded, but this is *not* implied. I.e. the application *must* use
  e=1 explicitly.

To load an icon (in-band PNG data):

    printf '\e]99;g=123:p=icon;<base64-encoded-png-data>\e\\'

or (symbolic name)

    printf '\e]99;g=123:n=firefox:p=icon;\e\\'

Of course, we can combine the two, assigning *both* a symbolic
name, *and* PNG data:

    printf '\e]99;g=123:n=firefox:p=icon;<base64-encoded-png>\e\\'

Then, to use the icon in a notification:

    printf '\e]99;g=123;this is a notification\e\\'

Foot also allows a *symbolic* icon to be defined and used at the same
time:

    printf '\e]99;g=123:n=firefox;this is a notification\e\\'

This obviously won't work with PNG data, since it uses the payload
portion of the escape sequence.

base64.c

@@ -42,7 +42,7 @@ static const char lookup[64] = {
 };
 
 char *
-base64_decode(const char *s)
+base64_decode(const char *s, size_t *size)
 {
     const size_t len = strlen(s);
     if (unlikely(len % 4 != 0)) {
@@ -54,6 +54,9 @@ base64_decode(const char *s)
     if (unlikely(ret == NULL))
         return NULL;
 
+    if (unlikely(size != NULL))
+        *size = len / 4 * 3;
+
     for (size_t i = 0, o = 0; i < len; i += 4, o += 3) {
         unsigned a = reverse_lookup[(unsigned char)s[i + 0]];
         unsigned b = reverse_lookup[(unsigned char)s[i + 1]];
@@ -68,6 +71,13 @@ base64_decode(const char *s)
             if (unlikely(i + 4 != len || (a | b) & P || (c & P && !(d & P))))
                 goto invalid;
 
+            if (unlikely(size != NULL)) {
+                if (c & P)
+                    *size = len / 4 * 3 - 2;
+                else
+                    *size = len / 4 * 3 - 1;
+            }
+
             c &= 63;
             d &= 63;
         }