VClearWindow - clears a region in the specified window.
VClearWindow(win, xloc, yloc, width, height)
int xloc, yloc;
int width, height;
VClearWindow clears a region in the specified window. If the window is
dialog window, VClearWindow clears the window or the pixmap depending on
whether the dialog window is in front or back of the image window.
If the window is the image window or one of its subwindows, then VClearWindow
clears the window itself. If the window is the button window, VClearWindow
clears the window itself. If the width and height are equal to 0, VClearWindow
clears the whole window from the location (xloc, yloc).
0 - work successfully.
6 - invalid window ID.
258 - pixmap of the dialog window is NULL and the image window is in
front of the dialog window.
win - specifies the ID of the window to be cleared.
xloc, yloc - specifies x and y coordinates of the upper left corner of the
rectangle, relative to the origin of the image window.
width, height - specifies the width and height of the window to be cleared.
If xloc, yloc, width, or height is not valid, or VCreateWindows is not
called earlier, VClearWindow prints an error message to the standard error
stream, produces a core dump file and exits from the current process.