Shuttle mouse support

[rattboi]

Closes #146

Details:

• Maps a USB mouse to p1/p2 Saturn Shuttle mouse

• Fixes joypad analog stick as mouse

• Merges inputs between the joypad and usb mouse, so you can swap between each on-the-fly

  • Maybe you like to use joypad for most of the game, but use the mouse for the shooting sections. Go for it.

• L/M/R USB mouse buttons map to Shuttle Mouse L/M/R buttons.

• Any extra USB mouse buttons detected are merged together as the 4th mouse button (Saturn Start).

  • for example, side buttons for page forward/page back

• Joypad analog as mouse is oversensitive just sending raw deltas, so it's scaled down to 37.5%.

  • Maybe this could be a sensitivity setting in the future.

[Zet-sensei]

Working fine (at least) with :

• Virtua Cop;
• Virtua Cop 2;
• Tokimeki Memorial - Forever with You;
• Sakura Taisen;
• Sakura Taisen 2;
• Dokyuusei:
• Dokyuusei if:
• Policenauts.

[rattboi]

https://github.com/rattboi/Saturn_MiSTer/raw/rattboi/mouse-support-rbf/releases/Saturn_mouse.rbf Adding RBF for easy access (not a part of the PR)