trace: Serialize wide-strings properly.

Wide strings are only used by DirectX APIs, mostly for descriptions, but
it's still useful to be able to read them.

I'm split on whether to bump the trace format version or not.
OpenGL traces are unaffected.
trace: Encode wide-strings as ASCII for now.

Just to avoid the hassle of introducing a trace format version bump.
dxgitrace: Avoid writing into Windows system dir.
common: Fix whitespace.