Returns the receiver's line join style, which will be one
of the constants <code>SWT.JOIN_MITER</code>, <code>SWT.JOIN_ROUND</code>,
or <code>SWT.JOIN_BEVEL</code>.
@return the join style used for drawing lines
@exception SWTException <ul>
<li>ERROR_GRAPHIC_DISPOSED - if the receiver has been disposed</li>
</ul>
Returns the receiver's line join style, which will be one of the constants <code>SWT.JOIN_MITER</code>, <code>SWT.JOIN_ROUND</code>, or <code>SWT.JOIN_BEVEL</code>.
@return the join style used for drawing lines
@exception SWTException <ul> <li>ERROR_GRAPHIC_DISPOSED - if the receiver has been disposed</li> </ul>
@since 3.1