If we are disabling WebRender, e.g. due to an init failure or GPU process crash, we should permit falling back to Software WebRender. It should only fallback where Software WebRender is already permitted for that user, i.e. it should honour the allowlist settings.

This should also consider cases where SW-WR + D3D11 compositor is being disabled; we either want to go to pure SW-WR or basic. If we are disabling SW-WR, make sure basic still works.

We can disable WebRender because the GPU process crashed, or we
encountered a graceful runtime error in WebRender. This patch adds two
new prefs to control how that fallback works.

gfx.webrender.fallback.software-d3d11 controls if WebRender falls back
to Software WebRender + D3D11 compositing. If true, and the user is
allowed to get Software WebRender, we will fallback to Software
WebRender with the D3D11 compositor first.

gfx.webrender.fallback.software controls if WebRender falls back to
Software WebRender. If true, and the user is allowed to get Software
WebRender, we will fallback to Software WebRender without the D3D11
compositor.

gfx.webrender.fallback.basic controls if WebRender or Software
WebRender falls back to Basic. If true, it falls back to Basic.
Otherwise it continues to use Software WebRender without the D3D11
compositor. Note that this means OpenGL on Android.

This patch also means that gfx.webrender.all=true and MOZ_WEBRENDER=1
no longer disables Software WebRender. It will still prefer (Hardware)
WebRender but we want to allow fallback to Software WebRender for
configurations that forced WebRender on.

Backed out for causing build bustages on GPUProcessManager.cpp.

Backout link: https://hg.mozilla.org/integration/autoland/rev/eef1077d4e44f24e82c86b6a54912d1df9f59a5d

Push with failures: https://treeherder.mozilla.org/jobs?repo=autoland&selectedTaskRun=Ec6FjhCpQBWMp9lGwQhuWw.0&revision=a3c9fce26dd175f84dd3f84a8d8b960bd63bfd11&searchStr=linux%2Cx64%2Copt%2Cbuild-linux64%2Fopt%2Cb

Failure log: https://treeherder.mozilla.org/logviewer?job_id=328491613&repo=autoland&lineNumber=36198

Backed out for causing failure on GfxConfigManager.

Backout link: https://hg.mozilla.org/integration/autoland/rev/54a94ce78e845198fe69629d8aca9537d774cbf2

Push with failures: https://treeherder.mozilla.org/jobs?repo=autoland&selectedTaskRun=RwkXF2aeTt-2LUgf0MkkXg.0&revision=7ba7ce64acae0c435fe10efcadc453b8b605cab3&searchStr=linux%2C18.04%2Cx64%2Cwebrender%2Copt%2Ctest-linux1804-64-qr%2Fopt-gtest-1proc%2Cgtest

Failure log: https://treeherder.mozilla.org/logviewer?job_id=328499157&repo=autoland&lineNumber=28490

https://hg.mozilla.org/mozilla-central/rev/8522d4a89bbd