avfilter/vf_subtitles: enable processing of alpha channel

Fixes #6605.
Signed-off-by: Paul B Mahol <onemda@gmail.com>

doc/filters.texi

@@ -14005,6 +14005,9 @@ correctly scale the fonts if the aspect ratio has been changed.
 Set a directory path containing fonts that can be used by the filter.
 These fonts will be used in addition to whatever the font provider uses.
 
+@item alpha
+Process alpha channel, by default alpha channel is untouched.
+
 @item charenc
 Set subtitles input character encoding. @code{subtitles} filter only. Only
 useful if not UTF-8.

libavfilter/vf_subtitles.c

@@ -54,6 +54,7 @@ typedef struct AssContext {
     char *charenc;
     char *force_style;
     int stream_index;
+    int alpha;
     uint8_t rgba_map[4];
     int     pix_step[4];       ///< steps per pixel for each plane of the main output
     int original_w, original_h;
@@ -69,6 +70,7 @@ typedef struct AssContext {
     {"f",              "set the filename of file to read",                         OFFSET(filename),   AV_OPT_TYPE_STRING,     {.str = NULL},  CHAR_MIN, CHAR_MAX, FLAGS }, \
     {"original_size",  "set the size of the original video (used to scale fonts)", OFFSET(original_w), AV_OPT_TYPE_IMAGE_SIZE, {.str = NULL},  CHAR_MIN, CHAR_MAX, FLAGS }, \
     {"fontsdir",       "set the directory containing the fonts to read",           OFFSET(fontsdir),   AV_OPT_TYPE_STRING,     {.str = NULL},  CHAR_MIN, CHAR_MAX, FLAGS }, \
+    {"alpha",          "enable processing of alpha channel",                       OFFSET(alpha),      AV_OPT_TYPE_BOOL,       {.i64 = 0   },         0,        1, FLAGS }, \
 
 /* libass supports a log level ranging from 0 to 7 */
 static const int ass_libavfilter_log_level_map[] = {
@@ -140,7 +142,7 @@ static int config_input(AVFilterLink *inlink)
 {
     AssContext *ass = inlink->dst->priv;
 
-    ff_draw_init(&ass->draw, inlink->format, 0);
+    ff_draw_init(&ass->draw, inlink->format, ass->alpha ? FF_DRAW_PROCESS_ALPHA : 0);
 
     ass_set_frame_size  (ass->renderer, inlink->w, inlink->h);
     if (ass->original_w && ass->original_h)