make it possible to enable fast-decode from the client via env var

git-svn-id: https://xpra.org/svn/Xpra/trunk@20397 3bb7dfac-3a0b-4e04-842a-767bc560f471

src/xpra/client/mixins/encodings.py

@@ -196,6 +196,7 @@ def get_encodings_caps(self):
                 profile = os.environ.get("XPRA_H264_%s_PROFILE" % (csc_name), default_profile)
                 if profile:
                     h264_caps["%s.profile" % (csc_name)] = profile
+            h264_caps["fast-decode"] = envbool("XPRA_X264_FAST_DECODE", False)
             log("x264 encoding options: %s", h264_caps)
             updict(caps, "h264", h264_caps)
         iq = max(self.min_quality, self.quality)