avcodec/ass: explicit PlayRes[XY]

This avoids the following libass warning when using the subtitles
filter: "Neither PlayResX nor PlayResY defined. Assuming 384x288"

Subtitles tests change because the output is ASS and the PlayRes[XY]
ends up in the output.

libavcodec/ass.c

@@ -34,6 +34,8 @@ int ff_ass_subtitle_header(AVCodecContext *avctx,
 {
     avctx->subtitle_header = av_asprintf(
              "[Script Info]\r\n"
+             "PlayResX: 384\r\n"
+             "PlayResY: 288\r\n"
              "ScriptType: v4.00+\r\n"
              "\r\n"
              "[V4+ Styles]\r\n"

tests/ref/fate/sub-aqtitle

-1c68def68db6536c235819cbe0638e00
+2603c926b8b235032762ef2ae47a7779

tests/ref/fate/sub-charenc

-9615088d613931b50fd5016c7535b99a
+6ae00c6970e873d82ef5d06d40e144fe

tests/ref/fate/sub-jacosub

-4e8f2d1d4130251163432d2a448680be
+5547b430172107fc242385e9c639fa93

tests/ref/fate/sub-microdvd

-35e133576aa3881d2de8dbf39a8d6df7
+c150177fc8bb928b51de3533fd105225

tests/ref/fate/sub-movtext

-21453e8ddbbe35d1368a99fe563c969d
+ac8c13bac8e0e43c84eaa525680964f0

tests/ref/fate/sub-mpl2

-3c2fb62002aec3af16d83135a0e3b0fc
+4a7699ff366788a4af6243ab28d2f9ff

tests/ref/fate/sub-mpsub

-2c5fafec41479e1d09a32f85e8927d03
+7d047bdf176d4477c8713e89be5dec8e

tests/ref/fate/sub-mpsub-frames

-cbe6e45848ef77e3080487a88b122104
+ae1dce6321a0eb1bc406ca8660350023

tests/ref/fate/sub-pjs

-d044f6ffdee48e48efff072b33baee0a
+1e188b18693844b3a30e91a650f68e1b

tests/ref/fate/sub-realtext

-5366f62be001f6fd4a7f48893828bf51
+77103e00504d1e1b3302a990a646ae38

tests/ref/fate/sub-sami

-00642e143339b4ca29dc6e990436387c
+4577f9c289440558026dc426584a06d8

tests/ref/fate/sub-srt

-a246a00ed72655cb8db3eaf64a522e0d
+5315582436b3381996dca4a767a080a0

tests/ref/fate/sub-subviewer

-aef995d49af4517b40589b72cfa918f7
+0954f324e9c8137d7f50fc705199e9a4

tests/ref/fate/sub-subviewer1

-cbeb015b1125757eed814f212cfc6c9c
+89aa422b7a6cd4dc9beef1b5a67d4aab

tests/ref/fate/sub-vplayer

-c8201c542f43a9ea42a787ac74d28049
+d8405387687269f7d763fc324c7c01bb

tests/ref/fate/sub-webvtt

-2cf38e2c99f8717f78a91a3f31197fb4
+6b915f0a00ab04649f6ce769da0af916