gfx 各个类的源码解读（6） WebGL2CommandBuffer

gfx 各个类的源码解读（6） WebGL2CommandBuffer
WebGL2CommandBuffer 在 webgl2-command-buffer.ts
对WebGL2CmdPackage类命令填充和状态保存,
WebGL2CommandAllocator 负责cmd的alloc，
上面两个类实例在成员变量里，不是全局的
成员变量：

    protected _cmdAllocator: WebGL2CommandAllocator = new WebGL2CommandAllocator();
    protected _cmdAllocator: WebGL2CommandAllocator = new WebGL2CommandAllocator();

状态保存成员变量（后面用到会详细解读）

    protected _curGPUPipelineState: IWebGL2GPUPipelineState | null = null;
    protected _curGPUDescriptorSets: IWebGL2GPUDescriptorSet[] = [];
    protected _curGPUInputAssembler: IWebGL2GPUInputAssembler | null = null;
    protected _curDynamicOffsets: number[] = Array(8).fill(0);
    protected _curDynamicStates: DynamicStates = new DynamicStates();
    protected _isStateInvalied = false;

比如对WebGL2CmdBeginRenderPass命令的填充

    public beginRenderPass (
        renderPass: RenderPass,
        framebuffer: Framebuffer,
        renderArea: Rect,
        clearColors: Color[],
        clearDepth: number,
        clearStencil: number,
    ) {
        //pool alloc cmd
        const cmd = this._cmdAllocator.beginRenderPassCmdPool.alloc(WebGL2CmdBeginRenderPass);  
        
        cmd.gpuRenderPass = (renderPass as WebGL2RenderPass).gpuRenderPass;
        cmd.gpuFramebuffer = (framebuffer as WebGL2Framebuffer).gpuFramebuffer;
        cmd.renderArea = renderArea;
        for (let i = 0; i < clearColors.length; ++i) {
            cmd.clearColors[i] = clearColors[i];
        }
        cmd.clearDepth = clearDepth;
        cmd.clearStencil = clearStencil;
        //cmdPackage装入cmd
        this.cmdPackage装入cmd.beginRenderPassCmds.push(cmd);
        this.cmdPackage.cmds.push(WebGL2Cmd.BEGIN_RENDER_PASS);
        this._isInRenderPass = true;
    }