m_property: add a way to switch on property values in property expansion

Allows for example: --status-msg='${?pause==yes:(Paused) } ...' to
emulate the normal terminal status line. It's useful in other situations
too.

I'm a bit worried about extending this mini-DSL, and sure hope nobody
will implement a generic formula evaluator at some point in the future.
But for now we're probably safe.

DOCS/man/en/input.rst

@@ -509,15 +509,27 @@ The following expansions are supported:
     fails, expand to an error string. (Use ``${NAME:}`` with a trailing
     ``:`` to expand to an empty string instead.)
     If ``NAME`` is prefixed with ``=``, expand to the raw value of the property
-    (see below).
+    (see section below).
 ``${NAME:STR}``
     Expands to the value of the property ``NAME``, or ``STR`` if the
     property cannot be retrieved. ``STR`` is expanded recursively.
+``${?NAME:STR}``
+    Expands to ``STR`` (recursively) if the property ``NAME`` is available.
 ``${!NAME:STR}``
     Expands to ``STR`` (recursively) if the property ``NAME`` cannot be
     retrieved.
-``${?NAME:STR}``
-    Expands to ``STR`` (recursively) if the property ``NAME`` is available.
+``${?NAME==VALUE:STR}``
+    Expands to ``STR`` (recursively) if the property ``NAME`` expands to a
+    string equal to ``VALUE``. You can prefix ``NAME`` with ``=`` in order to
+    compare the raw value of a property (see section below). If the property
+    is unavailable, or other errors happen when retrieving it, the value is
+    never considered equal.
+    Note that ``VALUE`` can't contain any of the characters ``:`` or ``}``.
+    Also, it is possible that escaping with ``"`` or ``%`` might be added in
+    the future, should the need arise.
+``${!NAME==VALUE:STR}``
+    Same as with the ``?`` variant, but ``STR`` is expanded if the value is
+    not equal. (Using the same semantics as with ``?``.)
 ``$$``
     Expands to ``$``.
 ``$}``

mpvcore/m_property.c

@@ -219,14 +219,20 @@ static int expand_property(const m_option_t *prop_list, char **ret, int *ret_len
 {
     bool cond_yes = bstr_eatstart0(&prop, "?");
     bool cond_no = !cond_yes && bstr_eatstart0(&prop, "!");
+    bool test = cond_yes || cond_no;
     bool raw = bstr_eatstart0(&prop, "=");
-    int method =
-        (raw || cond_yes || cond_no) ? M_PROPERTY_GET_STRING : M_PROPERTY_PRINT;
+    bstr comp_with = {0};
+    bool comp = test && bstr_split_tok(prop, "==", &prop, &comp_with);
+    if (test && !comp)
+        raw = true;
+    int method = raw ? M_PROPERTY_GET_STRING : M_PROPERTY_PRINT;
 
     char *s = NULL;
     int r = m_property_do_bstr(prop_list, prop, method, &s, ctx);
     bool skip;
-    if (cond_yes || cond_no) {
+    if (comp) {
+        skip = ((s && bstr_equals0(comp_with, s)) != cond_yes);
+    } else if (test) {
         skip = (!!s != cond_yes);
     } else {
         skip = !!s;