For GL backend, at first each worker thread must have a naitve context
for its own to work in. These worker contexts have to be shared from
the main context, so that all shader and program objects are seen in
any context. This extends backend displays to create and destroy the
worker contexts. RendererGL manages and allocates them to the worker
threads. ShaderImpl has a new compile method added to do the actual
glCompile work in worker thread. The ProgramGL's link method is broken
down by introducing the LinkEventGL class.

Bug: chromium:849576

Change-Id: Idc2c51b4b6c978781ae77810e62c480acc67ebb5
Reviewed-on: https://chromium-review.googlesource.com/c/1373015
Commit-Queue: Jie A Chen <jie.a.chen@intel.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>