options: make --ass-hinting a choice, instead of using magic numbers

options.c still doesn't use the constants defined by the libass headers,
but it's still better than exposing those to the user directly.

DOCS/man/en/options.rst

@@ -122,15 +122,20 @@
 
         Using this option may lead to incorrect subtitle rendering.
 
-``--ass-hinting=<type>``
+``--ass-hinting=none|light|normal|native``
     Set font hinting type. <type> can be:
 
-    :0:       no hinting
-    :1:       FreeType autohinter, light mode
-    :2:       FreeType autohinter, normal mode
-    :3:       font native hinter
+    :none:       no hinting (default)
+    :light:      FreeType autohinter, light mode
+    :normal:     FreeType autohinter, normal mode
+    :native:     font native hinter
 
-    The default value is 0 (no hinting).
+    .. admonition:: Warning
+
+        Enabling hinting can lead to mispositioned text (in situations it's
+        supposed to match up with video background), or reduce the smoothness
+        of animations with some badly authored ASS scripts. It is recommended
+        to not use this option, unless really needed.
 
 ``--ass-line-spacing=<value>``
     Set line spacing value for SSA/ASS renderer.

mpvcore/options.c

@@ -538,7 +538,8 @@ const m_option_t mp_opts[] = {
     OPT_FLAG("embeddedfonts", use_embedded_fonts, 0),
     OPT_STRINGLIST("ass-force-style", ass_force_style_list, 0),
     OPT_STRING("ass-styles", ass_styles_file, 0),
-    OPT_INTRANGE("ass-hinting", ass_hinting, 0, 0, 7),
+    OPT_CHOICE("ass-hinting", ass_hinting, 0,
+               ({"none", 0}, {"light", 1}, {"normal", 2}, {"native", 3})),
     OPT_CHOICE("ass-style-override", ass_style_override, 0,
                ({"no", 0}, {"yes", 1})),
     OPT_FLAG("osd-bar", osd_bar_visible, 0),

sub/ass_mp.c

@@ -132,7 +132,7 @@ void mp_ass_configure(ASS_Renderer *priv, struct MPOpts *opts,
 #endif
         set_line_spacing = opts->ass_line_spacing;
         set_font_scale = opts->sub_scale;
-        set_hinting = opts->ass_hinting & 3; // +4 was for no hinting if scaled
+        set_hinting = opts->ass_hinting;
     }
 
     ass_set_use_margins(priv, set_use_margins);