d3d12: Delete custom OpenGLOn12 target, add option to override libgallium_wgl.dll name
OpenGLOn12.dll is just libgallium_wgl.dll but built with a different name (and not supporting any other gallium drivers besides d3d12). This target should be deleted and we should add an option to just rename libgallium_wgl.dll. The only reason the rename is relevant is because Mesa's OpenGL32.dll links against the megadriver - if you ship the ICD standalone then you can just rename the file and it's fine, but if you want to use Mesa's GL frontend (e.g. because you're shipping it app-local instead of as a system ICD) then being stuck with libgallium_wgl.dll as a name kind of sucks.