[Win] Use automatic destruction for PbufferGLSurfaceWGL.

PbufferGLSurfaceWGL is a reference counted class, hence deleting
it manually triggers an assert in debug builds if Release is not
called or the reference count is non-zero.

Using a scoped_refptr prevents this and deletes the object
correctly.

Change-Id: I7e5ff783afc367e06d872b8d118c2ff83627b836
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>