ffmpeg 3.4 VDA (VideoDecodeAcceleration) fix

[RJVB (René Bertin)]

port:ffmpeg@3.4 has a regression in its VDA support, partly signalled in a comment in the Portfile.

I'm attaching a patchfile which fixes this as far as building the feature and exporting the associated symbols is concerned (as expected e.g. by VLC). I admit I haven't tested beyond that point (hardware video decoding support is flaky on my CPU/GPU anyway).

The patch needs a small modification to the Portfile which I'm copying inline here:

--- a/multimedia/ffmpeg/Portfile
+++ b/multimedia/ffmpeg/Portfile
@@ -187,7 +187,13 @@ platform darwin {
     # configure now fails on 10.11+ if --enable-vda is forced citing missing vda dependencies
     # if we just remove --disable-vda, ffmpeg will auto configure vda if the required dependencies are available
     # this allows ffmpeg to configure and build, effect on the bug cited above not verified
-    #   configure.args-append --enable-vda
+        if {${os.major} < 15} {
+            # we're on an OS version that has VDA. Enable it explicitly, and ensure that the
+            # appropriate symbols still get exported from libavcodec as they were in 3.3 and before.
+            configure.args-append --enable-vda
+            patchfiles-append \
+                              patch-fix-vda.diff
+        }
     }
 
     # AudiotoolBox support requires CoreMedia Framework available on 10.7+

Together the effect is:

1 enable VDA on 10.6.3 through 10.10 2 work around an issue in configure where it claims not to find vda_framework (it's part of the system so we don't need to check for it anyway after step 1) 3 patch the exported symbol generator file so that expected symbols are exported as they were in the past (on 10.6.3 through 10.10).

[mascguy (Christopher Nielsen)]

Reassign ffmpeg-related tickets to Jeremy