feat(后端架构): 完善GG后端，修复渐变与选区异常

ofdgo_renderer_canvas.go

@@ -58,7 +58,9 @@ func (b CanvasBackend) Render(page *RasterPage) (image.Image, error) {
 				}
 				source = converted
 			}
-			drawRasterImage(img, source, page.PixelTransform(cmd.Transform, h))
+			if err := drawRasterImageCommand(b, page, img, source, cmd); err != nil {
+				return nil, err
+			}
 			continue
 		}
 		p := &canvas.Path{}
@@ -93,6 +95,9 @@ func (b CanvasBackend) Render(page *RasterPage) (image.Image, error) {
 			default:
 				return nil, fmt.Errorf("canvas command %d: unsupported gradient %d", i, g.Kind)
 			}
+			if g.Spread != nil {
+				paint = canvas.Paint{Gradient: rasterGradientCanvas{g}}
+			}
 		}
 		style := canvas.Style{Fill: paint, FillRule: canvas.NonZero}
 		if cmd.EvenOdd {
@@ -104,3 +109,23 @@ func (b CanvasBackend) Render(page *RasterPage) (image.Image, error) {
 	r.Close()
 	return img, nil
 }
+
+// rasterGradientCanvas 将公共渐变交给Canvas采样，不改变OFD周期语义
+type rasterGradientCanvas struct{ gradient *RasterGradient }
+
+// At 返回局部坐标的预乘颜色
+// 入参: x 横坐标, y 纵坐标
+// 返回: color.RGBA 渐变颜色
+func (g rasterGradientCanvas) At(x, y float64) color.RGBA { return g.gradient.At(x, y) }
+
+// SetColorSpace 返回转换颜色空间后的独立渐变
+// 入参: space 目标颜色空间
+// 返回: canvas.Gradient 采样器
+func (g rasterGradientCanvas) SetColorSpace(space canvas.ColorSpace) canvas.Gradient {
+	copy := *g.gradient
+	copy.Stops = append([]ColorStop(nil), copy.Stops...)
+	for i := range copy.Stops {
+		copy.Stops[i].Color = space.ToLinear(copy.Stops[i].Color)
+	}
+	return rasterGradientCanvas{&copy}
+}