Wii IR scale: preserve stock cursor speed regardless of scale value

[Copilot]

WII_IR_SCALE was a plain linear multiplier applied uniformly across the screen — setting it above 1.0 made the cursor faster everywhere, not just at the edges. The ask is to keep stock (1:1) tracking speed at all times while still allowing the scale to control how far the cursor reaches.

Approach

Replaces the uniform linear formula with a cubic smooth-step position-dependent curve:

// Before — derivative == scale everywhere
finalX = 0.5 + (finalX - 0.5) * scale;

// After — derivative == 1.0 at centre, ramps to `scale` at the edge
double tx = fabs(finalX - 0.5) * 2.0;          // 0 at centre, 1 at edge
double wx = tx * tx * (3.0 - 2.0 * tx);        // smoothstep(0,1,t)
finalX = 0.5 + (finalX - 0.5) * (1.0 + (scale - 1.0) * wx);

• Centre derivative is always 1.0 — aiming speed in the middle of the screen is unchanged no matter what WII_IR_SCALE is set to.
• Scale ramps in toward the edges — coverage/reach is still extended (> 1.0) or contracted (< 1.0) as before.
• Fallback for scale < 0.5 — the smooth curve becomes non-monotone at extreme compression, so the original linear formula is kept below that threshold to prevent backwards cursor movement.

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