tablet: map BTN_STYLUS3 to button 8

Buttons 4-7 are out of bounds for hysterical historical reasons. Previously this button fell through to the default statement and resulted in 8 + BTN_STYLUS3 - BTN_SIDE == 65 which is rather obviously wrong.

Instead, map it explicitly to what the fourth button would be mapped to on other devices. This will now overlap with BTN_SIDE on devices that both BTN_STYLUS3 and BTN_SIDE but those devices don't appear to exist in the real world.

Fixes #50 (closed)

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