Log Message:
[libretro-]stella: update to 6.6

This is a new semi-major release, with the following changes:

    Added preliminary PlusROM support for saving high scores.

    Added preliminary support for 'MVC' bankswitching scheme by Rob Bairos.

    Added web links for many games.

    Added dead zone and linearity settings for analog controllers.

    Added 'Check for Update' button to Help dialog.

    Added different mask patterns for scanline emulation.

    Fixed MindLink controller.

    Fixed SaveKey not working with QuadTari.

    Added TIA randomization on startup option.

    Added different debug color luminances for player and missile copies.

    Added hotkeys for TV roll speed and toggling 'Developer settings' sets.

    Debugger: enhanced prompt's auto complete and history.

    Debugger: added optional logging of breaks and traps.

    Debugger: added Thumb cycle counting.