Added helpers to create text subpicture.

It creates a subpicture rendering text at the display size.

modules/codec/substext.h

+struct subpicture_updater_sys_t {
+    char *text;
+    char *html;
+
+    int  align;
+    int  x;
+    int  y;
+
+    bool is_fixed;
+    int  fixed_width;
+    int  fixed_height;
+};
+
+static int SubpictureTextValidate(subpicture_t *subpic,
+                                  bool has_src_changed, const video_format_t *fmt_src,
+                                  bool has_dst_changed, const video_format_t *fmt_dst,
+                                  mtime_t ts)
+{
+    subpicture_updater_sys_t *sys = subpic->updater.p_sys;
+    VLC_UNUSED(fmt_src); VLC_UNUSED(fmt_dst); VLC_UNUSED(ts);
+
+    if (!has_src_changed && !has_dst_changed)
+        return VLC_SUCCESS;
+    if (!sys->is_fixed && subpic->b_absolute && subpic->p_region &&
+        subpic->i_original_picture_width > 0 &&
+        subpic->i_original_picture_height > 0) {
+
+        sys->is_fixed     = true;
+        sys->x            = subpic->p_region->i_x;
+        sys->y            = subpic->p_region->i_y;
+        sys->fixed_width  = subpic->i_original_picture_width;
+        sys->fixed_height = subpic->i_original_picture_height;
+    }
+    return VLC_EGENERIC;
+}
+static void SubpictureTextUpdate(subpicture_t *subpic,
+                                 const video_format_t *fmt_src,
+                                 const video_format_t *fmt_dst,
+                                 mtime_t ts)
+{
+    subpicture_updater_sys_t *sys = subpic->updater.p_sys;
+    VLC_UNUSED(fmt_src); VLC_UNUSED(ts);
+
+    if (fmt_dst->i_sar_num <= 0 || fmt_dst->i_sar_den <= 0)
+        return;
+
+    subpic->i_original_picture_width  = fmt_dst->i_width * fmt_dst->i_sar_num / fmt_dst->i_sar_den;
+    subpic->i_original_picture_height = fmt_dst->i_height;
+
+    video_format_t fmt;
+    video_format_Init(&fmt, VLC_CODEC_TEXT);
+    fmt.i_sar_num = 1;
+    fmt.i_sar_den = 1;
+
+    subpicture_region_t *r = subpic->p_region = subpicture_region_New(&fmt);
+    if (!r)
+        return;
+
+    r->psz_text = sys->text ? strdup(sys->text) : NULL;
+    r->psz_html = sys->html ? strdup(sys->html) : NULL;
+    r->i_align  = sys->align;
+    if (!sys->is_fixed) {
+        const float margin_ratio = 0.04;
+        const int   margin_h     = margin_ratio * fmt_dst->i_visible_width;
+        const int   margin_v     = margin_ratio * fmt_dst->i_visible_height;
+
+        r->i_x = 0;
+        if (r->i_align & SUBPICTURE_ALIGN_LEFT)
+            r->i_x += margin_h + fmt_dst->i_x_offset;
+        else if (r->i_align & SUBPICTURE_ALIGN_RIGHT)
+            r->i_x += margin_h + fmt_dst->i_width - (fmt_dst->i_visible_width + fmt_dst->i_x_offset);
+
+        r->i_y = 0;
+        if (r->i_align & SUBPICTURE_ALIGN_TOP )
+            r->i_y += margin_v + fmt_dst->i_y_offset;
+        else if (r->i_align & SUBPICTURE_ALIGN_BOTTOM )
+            r->i_y += margin_v + fmt_dst->i_height - (fmt_dst->i_visible_height + fmt_dst->i_y_offset);
+    } else {
+        /* FIXME it doesn't adapt on crop settings changes */
+        r->i_x = sys->x * fmt_dst->i_width  / sys->fixed_width;
+        r->i_y = sys->y * fmt_dst->i_height / sys->fixed_height;
+    }
+}
+static void SubpictureTextDestroy(subpicture_t *subpic)
+{
+    subpicture_updater_sys_t *sys = subpic->updater.p_sys;
+
+    free(sys->text);
+    free(sys->html);
+    free(sys);
+}
+
+static inline subpicture_t *decoder_NewSubpictureText(decoder_t *decoder)
+{
+    subpicture_updater_sys_t *sys = calloc(1, sizeof(*sys));
+    subpicture_updater_t updater = {
+        .pf_validate = SubpictureTextValidate,
+        .pf_update   = SubpictureTextUpdate,
+        .pf_destroy  = SubpictureTextDestroy,
+        .p_sys       = sys,
+    };
+    subpicture_t *subpic = decoder_NewSubpicture(decoder, &updater);
+    if (!subpic)
+        free(sys);
+    return subpic;
+}