Add support for SamplerState.MaxMipLevel (WindowsGL and iOS)

SamplerState.MaxMipLevel allows to limit the maximum mipmap (the smallest one) used when mipmap filtering is enabled.
This adds support for it on WindowsGL (native support) and iOS (i am not sure there are any equivalent extension from other vendors).

This has been tested in production for a while in our branch; i am slowly bringing back all the changes we made since our last fork.