Windows GStreamer DLL packaging needs to include more plugins

[jdm]

From an email with friends at Centricular:

> https://github.com/servo/servo/blob/8e783577d24cac2983fbea1b46d5452ffa5fb44f/python/servo/build_commands.py#L737-L759
>

That lists all the libraries that Servo links to, but it doesn't list
the plugins that are loaded at runtime. For UWP you'll need to add
those as assets alongside the libraries. I grepped the servo/media
source:

grep -rie gst::ElementFactory::make * | sed -ne 's/.*gst::ElementFactory::make("\([^"]*\)".*/\1/p' | sort | uniq

This yielded the following elements:

appsink
appsrc
audioconvert
audioresample
audiotestsrc
autoaudiosink
capsfilter
decodebin
deinterleave
glsinkbin
opusenc
proxysink
proxysrc
queue
rtpopuspay
rtpvp8pay
videoconvert
videotestsrc
vp8enc
webrtcbin

These correspond to the following plugins:

gstapp
gstaudioconvert
gstaudioresample
gstaudiotestsrc
gstautodetect
gstcoreelements
gstplayback
gstinterleave
gstopengl
gstopus (*)
gstproxy
gstrtp
gstvideoconvert
gstvideotestsrc
gstvpx (*)
gstwebrtc (*)

And:

I forgot to mention another thing: autovideosink, autoaudiosink
(gstautodetect), and decodebin (gstplayback) will autodetect which
elements can / should be used for the pipeline and load them
dynamically. So you'll also need at least wasapisink (gstwasapi),
glimagesink (gstopengl) and avdec_{h264,vp8,aac,opus,*} (gstlibav). 

[jdm]

We should do a few things here:

• increase the set of packaged DLLs to include the plugins that live in $gst_root/libs/gstreamer-1.0
• add an explicit step when initializing the media backend to load the packaged plugins into the gstreamer registry

This will allow local desktop builds to work as well as nightly builds.