video: allow hardware decoding only for certain codecs

In particular, this disables mpeg4. There are some files out there that
use GMC, a usually rarely used and ineffective feature, which is not
supported by most hardware decoders. In these cases the hw decoder
outputs garbage, while software decoding works perfectly fine. We can't
really fallback to software decoding in these cases, because we don't
know that something is wrong in the first place. I can't see any
advantages of hw decoding of mpeg4, so it's better to disable it.

DOCS/man/en/options.rst

@@ -1135,12 +1135,20 @@ OPTIONS
     The ``vaapi-copy`` function allows you to use vaapi with any VO. Because
     this copies the decoded video back to system RAM, it's quite inefficient.
 
-``--hwdec-codecs=<codec1,codec2,...|all>``
-    Allow hardware decoding for a given list of codecs only. The default is the
-    special value ``all``, which always allows all codecs.
+    .. note::
 
-    This is usually only needed with broken GPUs, where fallback on software
-    decoding does not work properly.
+        When using this switch, hardware decoding is still only done for some
+        codecs. See ``--hwdec-codecs`` to enable hardware decoding for more
+        codecs.
+
+``--hwdec-codecs=<codec1,codec2,...|all>``
+    Allow hardware decoding for a given list of codecs only. The special value
+    ``all`` always allows all codecs.
+
+    By default this is set to ``h264,vc1,wmv3``.
+
+    This is usually only needed with broken GPUs, where a codec is reported
+    as supported, but decoding causes more problems than it solves.
 
     .. admonition:: Example
 

mpvcore/options.c

@@ -851,7 +851,7 @@ const struct MPOpts mp_default_opts = {
     .sub_cp = "UTF-8:UTF-8-BROKEN",
 #endif
 
-    .hwdec_codecs = "all",
+    .hwdec_codecs = "h264,vc1,wmv3",
 
     .index_mode = -1,