Support running wrench with ANGLE on Windows.

[nical]

It should prove handy whenever we'll get to debugging ANGLE/D3D specific bugs, and D3D11 tends to have much better tooling available than GL, for example pixel history and debugging in renderdoc.

[kvark]

Pretty sure this is already done. Wrench uses glutin for initialization, and that one checks for libEGL as of servo/glutin#112 . All you need is to drop Angle's libEGL.dll in the folder with wrench executable.

[glennw]

This is a duplicate of #1432.