Intent to build (but not enable) WebRender by default on some Nightly platforms

[Kartikaya Gupta]

(Cross-posted to dev-platform and dev-tech-gfx, please reply to dev-platform)

In the near future I'm planning to make a change so that WebRender
will be built (but not enabled) by default in most Nightly/local
builds [1]. This will allow anybody running a stock nightly to turn on
The QuantumRender Experience™ by flipping a pref [2] and restarting
the browser [3].

Because this change will build more rust code into libxul,
compilation/linking time may be negatively impacted. Developers will
still be able to explicitly disable building webrender by putting
`ac_add_options --disable-webrender` in their mozconfig.

Furthermore, explicitly putting `ac_add_options --enable-webrender`
into your mozconfig will enable the pref for you by default, so that
if you have been doing builds with this flag already and expecting QR
to be on by default, it will continue to do so.

Patches are in bug 1342450 and dependencies if you want to follow
along. If you run into problems with this, please file a bug and cc
me, or reply to this email.

Cheers,
kats

[1] Not yet on Android, or OS X automation builds on Buildbot
[2] The pref is gfx.webrender.enabled
[3] There are more conditions in play than just the pref, see
gfxPlatform::InitWebRenderConfig().

[Kartikaya Gupta]

Note that this landed on inbound yesterday and was merged to m-c this
morning. If you get strange build errors (bkelly already reported one
due to missing the glXGetCurrentContext symbol) it might be because of
this. Adding ac_add_options --disable-webrender to your mozconfig
should work around it, but please file a bug as well and mark it
blocking bug 1342450. Thanks!