glViewport — set the viewport
| void glViewport( | GLint x, | 
| GLint y, | |
| GLsizei width, | |
| GLsizei height ); | 
x, ySpecify the lower left corner of the viewport rectangle, in pixels. The initial value is (0,0).
width, height
                    Specify the width and height
                    of the viewport.
                    When a GL context is first attached to a window,
                    width and height are set to the dimensions of that
                    window.
                
            glViewport specifies the affine transformation of 
            
            Viewport width and height are silently clamped
            to a range that depends on the implementation.
            To query this range, call glGet with argument
            GL_MAX_VIEWPORT_DIMS.
        
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