Add missing context3 for Windows. Add missing d3dcompiler_47.dll loading. Check whether dll architecture matches. Search ANGLE from Chrome path. Fixes golang/go#16991 Change-Id: Ia042f75241c2398fabda03bb2d0e683eb34545c7 Reviewed-on: https://go-review.googlesource.com/28814 Reviewed-by: David Crawshaw <crawshaw@golang.org> Reviewed-by: Alex Brainman <alex.brainman@gmail.com>