OpenGL config: clear backend_info.Adapters on init

Fixes a minor UI glitch where if you switch from Direct3D to OpenGL, the
OpenGL config page would appear to have an adapter choice available.
Merge pull request #264 from magumagu/opengl-clear-adapters

OpenGL: Clear backend_info.Adapters on initialization
VideoSoftware: remove const memory usage of DebugUtil
Merge pull request #135 from degasus/videosw_debugutil

VideoSoftware: Remove const memory usage of DebugUtil