ffmpeg_opt: rename loop option to stream_loop

The "loop" option is used in several demuxers (like img2dec) and muxers, using the same name in ffmpeg_opt
breaks them. Feel free to revert this and replace by any other solution or rename both as preferred
This is just as a quick fix to avoid the regression with existing command lines and to have both named
the same (which does not work)

Example:
./ffmpeg -loop 1 -i fate-suite/png1/lena-rgb24.png -t 1 test.avi
will produce 25 frames with the img2dec loop but only 1 frame at 25fps with the ffmpeg loop option
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

ffmpeg_opt.c

@@ -3158,7 +3158,7 @@ const OptionDef options[] = {
     { "dump_attachment", HAS_ARG | OPT_STRING | OPT_SPEC |
                          OPT_EXPERT | OPT_INPUT,                     { .off = OFFSET(dump_attachment) },
         "extract an attachment into a file", "filename" },
-    { "loop", OPT_INT | HAS_ARG | OPT_EXPERT | OPT_INPUT |
+    { "stream_loop", OPT_INT | HAS_ARG | OPT_EXPERT | OPT_INPUT |
                         OPT_OFFSET,                                  { .off = OFFSET(loop) }, "set number of times input stream shall be looped", "loop count" },
     { "debug_ts",       OPT_BOOL | OPT_EXPERT,                       { &debug_ts },
         "print timestamp debugging info" },