feat(后端架构): 优化多后端渲染，修复描边与字形校验

ofdgo_raster_scene.go

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+// Copyright 2025-2026 肖其顿 (XIAO QI DUN)
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package ofdgo
+
+import (
+	"fmt"
+	"image"
+	"slices"
+)
+
+// RasterScene 固定尺寸、DPI、字体和资源的可重复绘制场景
+// 每次Render返回独立图像；源图像只读借用，修改页面或配置后应重新准备
+type RasterScene interface {
+	Render() (image.Image, error)
+}
+
+// RasterSceneBackend 可选的光栅预编译能力，不改变后端选择
+type RasterSceneBackend interface {
+	RasterBackend
+	Prepare(page *RasterPage) (RasterScene, error)
+}
+
+// PreparePage 编译并准备当前配置的页面场景，不自动跟踪后续编辑
+// 入参: page 源页面
+// 返回: RasterScene 可重复绘制场景, error 编译或准备错误
+func (r *Renderer) PreparePage(page *PageContent) (RasterScene, error) {
+	if r.backends.Raster == nil {
+		return nil, fmt.Errorf("raster scene: %w", ErrBackendUnavailable)
+	}
+	compiled, err := r.CompilePage(page)
+	if err != nil {
+		return nil, err
+	}
+	return PrepareRasterScene(r.backends.Raster, compiled)
+}
+
+// PrepareRasterScene 准备绘制快照，未实现预编译的后端复用公共指令
+// 入参: backend 光栅后端, page 绘制页面
+// 返回: RasterScene 可重复绘制场景, error 准备错误
+func PrepareRasterScene(backend RasterBackend, page *RasterPage) (RasterScene, error) {
+	if backend == nil {
+		return nil, fmt.Errorf("raster scene: %w", ErrBackendUnavailable)
+	}
+	if _, _, err := page.PixelSize(); err != nil {
+		return nil, err
+	}
+	if provider, ok := backend.(RasterSceneBackend); ok {
+		return provider.Prepare(page)
+	}
+	return &rasterScene{backend: backend, page: cloneRasterPage(page)}, nil
+}
+
+// rasterScene 复用后端无关指令，图片保持只读引用
+type rasterScene struct {
+	backend RasterBackend
+	page    *RasterPage
+}
+
+// Render 绘制固定快照
+// 返回: image.Image 独立图像, error 绘制错误
+func (s *rasterScene) Render() (image.Image, error) { return s.backend.Render(s.page) }
+
+// cloneRasterPage 隔离可变指令，源图片按接口约定只读借用
+// 入参: page 绘制页面
+// 返回: *RasterPage 指令快照
+func cloneRasterPage(page *RasterPage) *RasterPage {
+	result := *page
+	result.Commands = slices.Clone(page.Commands)
+	for i := range result.Commands {
+		cmd := &result.Commands[i]
+		cmd.Path, cmd.Clip = slices.Clone(cmd.Path), slices.Clone(cmd.Clip)
+		if cmd.Stroke != nil {
+			stroke := *cmd.Stroke
+			stroke.Dashes = slices.Clone(stroke.Dashes)
+			cmd.Stroke = &stroke
+		}
+		if cmd.Paint.Gradient != nil {
+			gradient := *cmd.Paint.Gradient
+			gradient.Stops = slices.Clone(gradient.Stops)
+			if gradient.Spread != nil {
+				spread := *gradient.Spread
+				gradient.Spread = &spread
+			}
+			cmd.Paint.Gradient = &gradient
+		}
+	}
+	return &result
+}