Fix precision loss in DrawLine and DrawLineAa for large bitmaps

[Copilot]

Integer division and insufficient bit precision cause endpoint errors on large bitmaps. Drawing from (0,0) to (29999,9999) on a 30000×10000 bitmap produces a 39-pixel error in DrawLine and 1-pixel error in DrawLineAa.

Changes

DrawLine

• Add EXTRA_PRECISION_SHIFT = 16 for slope calculations (24-bit total precision vs 8-bit)
• Change slope and accumulator variables from int to long to prevent overflow
• Apply to both x-dominant and y-dominant code paths

DrawLineAa

• Increase PRECISION_SHIFT from 10 to 18 to prevent invD rounding to zero
• Add overflow protection: (long)du * du + (long)dv * dv
• Promote intermediate calculation variables to long

Root Causes

DrawLine: Slope calculation (dy << 8) / dx loses fractional bits. For dy=9999, dx=29999, true slope is 85.326 but truncates to 85. Over 30000 iterations: error = 0.326 × 30000 / 256 ≈ 39 pixels.

DrawLineAa: For long lines, invDFloat ≈ 0.000008. At PRECISION_SHIFT=10, (int)(0.000008 × 1024) = 0, breaking anti-aliasing. At PRECISION_SHIFT=18, (int)(0.000008 × 262144) = 2.

// Before (loses precision)
int incy = (dy << PRECISION_SHIFT) / dx;  // 8-bit precision

// After (maintains precision)
const int EXTRA_PRECISION_SHIFT = 16;
long incy = ((long)dy << (PRECISION_SHIFT + EXTRA_PRECISION_SHIFT)) / dx;  // 24-bit precision

Original prompt

---

This section details on the original issue you should resolve

<issue_title>DrawLine and DrawLineAa don't draw correctly on larger WriteableBitmaps</issue_title>
<issue_description>Drawing a line over large distances with DrawLine, the end will be off. In the example below there is a difference of 39 pixels in height.
Drawing a line with DrawLineAa, the end will be of by 1 in width and height.

 WriteableBitmap image = new WriteableBitmap(30000, 10000, 96, 96, PixelFormats.Pbgra32, null);
 image.Clear(Colors.White);
 image.DrawLine(0, 0, 29999, 9999, Colors.Black);
 image.DrawLineAa(0, 0, 29999, 9999, Colors.Red);

</issue_description>

<agent_instructions>First validate the issue and then go ahead and fix it. Make sure to keep performance in mind and only make minimal changes. Take your time and test all well. </agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes DrawLine and DrawLineAa don't draw correctly on larger WriteableBitmaps #115

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