avcodec: Add "sar" alias to "aspect" option of video encoders

It is impossible to pass "aspect" parameter to encoder from ffmpeg CLI
because option from lavc/options_table.h is eclipsed by option with same
name in ffmpeg_opt.c, which has different meaning (DAR, not SAR).
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

doc/codecs.texi

@@ -456,6 +456,9 @@ Possible values:
 @item aspect @var{rational number} (@emph{encoding,video})
 Set sample aspect ratio.
 
+@item sar @var{rational number} (@emph{encoding,video})
+Set sample aspect ratio. Alias to @var{aspect}.
+
 @item debug @var{flags} (@emph{decoding/encoding,audio,video,subtitles})
 Print specific debug info.
 

libavcodec/options_table.h

@@ -254,6 +254,7 @@ static const AVOption avcodec_options[] = {
 {"median", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_PRED_MEDIAN }, INT_MIN, INT_MAX, V|E, "pred"},
 #endif
 {"aspect", "sample aspect ratio", OFFSET(sample_aspect_ratio), AV_OPT_TYPE_RATIONAL, {.dbl = 0}, 0, 10, V|E},
+{"sar",    "sample aspect ratio", OFFSET(sample_aspect_ratio), AV_OPT_TYPE_RATIONAL, {.dbl = 0}, 0, 10, V|E},
 {"debug", "print specific debug info", OFFSET(debug), AV_OPT_TYPE_FLAGS, {.i64 = DEFAULT }, 0, INT_MAX, V|A|S|E|D, "debug"},
 {"pict", "picture info", 0, AV_OPT_TYPE_CONST, {.i64 = FF_DEBUG_PICT_INFO }, INT_MIN, INT_MAX, V|D, "debug"},
 {"rc", "rate control", 0, AV_OPT_TYPE_CONST, {.i64 = FF_DEBUG_RC }, INT_MIN, INT_MAX, V|E, "debug"},

tests/ref/fate/api-mjpeg-codec-param

@@ -62,6 +62,7 @@ stream=0, decode=0
     bits_per_coded_sample=0
     pred=0
     aspect=180/180
+    sar=180/180
     debug=0x00000000
     vismv=0x00000000
     cmp=0
@@ -218,6 +219,7 @@ stream=0, decode=1
     bits_per_coded_sample=0
     pred=0
     aspect=180/180
+    sar=180/180
     debug=0x00000000
     vismv=0x00000000
     cmp=0

tests/ref/fate/api-png-codec-param

@@ -62,6 +62,7 @@ stream=0, decode=0
     bits_per_coded_sample=0
     pred=0
     aspect=2835/2835
+    sar=2835/2835
     debug=0x00000000
     vismv=0x00000000
     cmp=0
@@ -218,6 +219,7 @@ stream=0, decode=1
     bits_per_coded_sample=0
     pred=0
     aspect=2835/2835
+    sar=2835/2835
     debug=0x00000000
     vismv=0x00000000
     cmp=0