PSA: WR code is now in gfx/wr

[Kartikaya Gupta]

Bug 1507524 (currently in the landing queue) moves the WR code from
being spread across gfx/webrender, gfx/webrender_api, and gfx/wrench
into gfx/wr. It also adds all the remaining stuff from the webrender
repo that wasn't there before.

For the most part the existing workflows should remain unchanged - WR
PRs that are inflight should be unaffected, and I'll continue to land
WR updates from servo/webrender into m-c as before.

There are only two real workflow changes:

(1) If you develop on standalone webrender, and copy your changes into
a mozilla tree to do try pushes, you now have to copy your changes
into gfx/wr instead of the gfx/webrender* folders. Here's a one-liner
that should work:
rm -rf $MOZILLA/gfx/wr && cp -R $WEBRENDER $MOZILLA/gfx/wr && rm -rf
$MOZILLA/gfx/wr/.git

(2) If you develop WR changes in a mozilla tree and then export them
to a WR tree to make PRs, you should now work in gfx/wr and export all
changes in that folder. I've updated the wiki page at [1] with
"before" and "after" commands for this workflow.

Existing try syntax etc should all be the same. If anybody runs into a
problem please let me know.

Cheers,
kats

[1] https://wiki.mozilla.org/Platform/GFX/Quantum_Render#Contribution_workflow