Crop a sub-window from an image

% solve the boundary problem when cropping RECT in IM
% RES = subwin(IM, RECT)
%
% INPUT
% IM: input image
% RECT: specific rectangle with coordinates (x, y, w, h)
%
% OUTPUT
% RES: cropped subwindow
%
% EXAMPLE
% im = imread('test.png');
% imshow(subwin(im, [-10, -10, 100, 100]));
%