How to exclude a rectangular area from another rectangular area

[li han]

Similar to this
```c++
Gdiplus::Graphics graphics(dcCompatible);

HRGN hgn1 = CreateRectRgn(m_rectTracker.m_rect.left,m_rectTracker.m_rect.top,
m_rectTracker.m_rect.right,m_rectTracker.m_rect.bottom);
Region region1(hgn1);

HRGN hgn2 = CreateRectRgn(rect.left,rect.top,
rect.right,rect.bottom);
Region region2(hgn2);

region2.Exclude(&region1);

SolidBrush  solidBrush(Color(100, 128, 128, 128));
graphics.FillRegion(&solidBrush,&region2);

DeleteObject(hgn1);
DeleteObject(hgn2);
```

[Manolo]

If what you want to do with the HRGN object is to use it as a clip region, yes, you can

use FLTK function fl_clip_region(Fl_Region r)

see https://www.fltk.org/doc-1.3/group__fl__drawings.html#gab12cb4aaa22092a0f2ae3d349c57f234

It uses the operating system-specific type Fl_Region which is exactly HRGN under Windows.