fix: ImageViewer => opacity handling for layer rectangles of context items

Even in 'image_viewer.hpp' context items need no layer fill boxes with opacity, like in 'image_viewer.cpp'.

tool/demo for testing: "pcl_pcd_select_object_plane" with input file: "trunk/test/milk_cartoon_all_small_clorox.pcd"

before:
![imageviewer - before](https://cloud.githubusercontent.com/assets/4068805/5515017/29b712ba-8857-11e4-98f5-f51f13796796.jpg)

after:
![imageviewer - after](https://cloud.githubusercontent.com/assets/4068805/5515018/3434316e-8857-11e4-8cce-71c030c9b663.jpg)

for better visibility the opacity of the added planar polygon was set to '0.1' (pcl_pcd_select_object_plane.cpp, line 468):
![pcd_select_object_plane](https://cloud.githubusercontent.com/assets/4068805/5515035/dff5ba22-8857-11e4-8b17-252bcc61e331.jpg)

See also the associated issues #998 and #1049.