Bug: Incorrect Gradient for Off-Diagonal Conic Term in Backward Pass

[OsamaS99]

The gradient for the off-diagonal element of the 2D conic matrix in gauss_renderer/cuda_rasterizer/backward.cu is incorrect. It is scaled by 0.5, making it half of the correct value derived from the forward pass calculation.

• Forward Pass (forward.cu): power = -0.5f * (a*dx² + c*dy²) - b*dx*dy;
• Correct Derivative of power w.r.t b: -dx*dy
• Backward Pass (backward.cu): The code incorrectly uses -0.5f * dx*dy to compute the gradient for b.

Change the coefficient for the y component of dL_dconic2D from -0.5f to -1.0f.

- atomicAdd(&dL_dconic2D[global_id].y, -0.5f * gdx * d.y * dL_dG);
+ atomicAdd(&dL_dconic2D[global_id].y, -1.0f * gdx * d.y * dL_dG);