sub: sub-filter-regex and jsre: support ass-to-plaintext

Using --sub-filter-regex-plain (default:no)

The ass-to-plaintext functionality already existed at sd_ass.c, but
it's internal and uses a private buffer type, so a trivial utility
wrapper was added with standard char*/bstr interface.

The plaintext can be multi-line, and the multi-line regexp flag is now
always set, but only affects plaintext (the ASS source is one line).

options/options.c

@@ -218,6 +218,7 @@ const struct m_sub_options mp_sub_filter_opts = {
         {"sub-filter-sdh", OPT_FLAG(sub_filter_SDH)},
         {"sub-filter-sdh-harder", OPT_FLAG(sub_filter_SDH_harder)},
         {"sub-filter-regex-enable", OPT_FLAG(rf_enable)},
+        {"sub-filter-regex-plain", OPT_FLAG(rf_plain)},
         {"sub-filter-regex", OPT_STRINGLIST(rf_items)},
         {"sub-filter-jsre", OPT_STRINGLIST(jsre_items)},
         {"sub-filter-regex-warn", OPT_FLAG(rf_warn)},

options/options.h

@@ -114,6 +114,7 @@ struct mp_sub_filter_opts {
     int sub_filter_SDH;
     int sub_filter_SDH_harder;
     int rf_enable;
+    int rf_plain;
     char **rf_items;
     char **jsre_items;
     int rf_warn;