webgl: Unlock WebRender by default by adding a readback based fallback to WebGL context creation.
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This should give us the chance to use WebRender by default in OSX.
r? @pcwalton
cc @glennw @larsbergstrom
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WebRender is an experimental GPU accelerated rendering backend for Servo.
The WebRender backend can be specified by running Servo with the -w option (otherwise the default rendering backend will be used).
WebRender has many bugs, and missing features - but it is usable to browse most websites - please report any WebRender specific rendering bugs you encounter!
