Sets the receiver's line cap style to the argument, which must be one
of the constants <code>SWT.CAP_FLAT</code>, <code>SWT.CAP_ROUND</code>,
or <code>SWT.CAP_SQUARE</code>.
@param cap the cap style to be used for drawing lines
@exception IllegalArgumentException <ul>
<li>ERROR_INVALID_ARGUMENT - if the style is not valid</li>
</ul>
@exception SWTException <ul>
<li>ERROR_GRAPHIC_DISPOSED - if the receiver has been disposed</li>
</ul>
Sets the receiver's line cap style to the argument, which must be one of the constants <code>SWT.CAP_FLAT</code>, <code>SWT.CAP_ROUND</code>, or <code>SWT.CAP_SQUARE</code>.
@param cap the cap style to be used for drawing lines
@exception IllegalArgumentException <ul> <li>ERROR_INVALID_ARGUMENT - if the style is not valid</li> </ul> @exception SWTException <ul> <li>ERROR_GRAPHIC_DISPOSED - if the receiver has been disposed</li> </ul>
@since 3.1