app: GimpRectangleTool: Rounding error when moving/resizing with keyboard

Fix for bug #653186: When a rectangle selection is created or moved by
mouse at certain zoom-levels (200%, 400%, 800%), its coordinates can
become exactly .500. By then using the keyboard to move/resize in
steps of one pixel, a rounding error occurs in
gimp_rectangle_tool_update_int_rect(). This leads to the coordinate
either not beeing changed at all or changed by 2 pixels at a time. The
patch changes the function from using RINT() to ROUND() for the
calculations, which prevents the rounding error from happening.