[WTL]部分DC操作

1. SaveDC和RestoreDC.

说起来很费解，不如写个代码show下它们的能力~

// DCLib... ...
void Draw(HDC hdc)
{
    // Draw a line (10, 10) - (190, 190)
    MoveToEx(hdc, 10, 10, (LPPOINT)NULL);
    LineTo(hdc, 190, 190);

    // comments? [2]
    SaveDC(hdc);

    // comments? [1]
    IntersectClipRect(hdc, 20, 20, 100, 100);

    // Draw a rect [(10, 10) - (30, 30)]
    RECT r = { 10, 10, 30, 30};
    HBRUSH b = (HBRUSH)CreateSolidBrush(RGB(255, 0, 255));
    FillRect(hdc, &r, b);

    // comments? [2]
    RestoreDC(hdc, -1);

    MoveToEx(hdc, 20, 10, (LPPOINT)NULL);
    LineTo(hdc, 190, 190);

    
    int a = 0;
}

2. IntersectClipRect

可以用IntersectClipRect设定DC的可绘制范围(矩形)，但是被Clip掉一部分的事实，怎么才能被Clear掉呢。

用SelectClipRgn(hdc, NULL);即可。