Revert "OpenGL mipmap fix for Windows."

This reverts commit 04474fa3766d955d6379e81cc7f6f7520c32cf2a.
Ogre Renderer and assets: Implement top level asynch checks that must be applied to all Ogre IAsset implementations. Apply IAsset specific rules after it. Cleanup the async detection code to be simpler and more readable. Major change is that we never allo async loading on windows+opengl combo, as it does not work there.