Correct colored overlays display in OpenGL mode

It started with my search to display BSH files (#FS1056)
As it is, Grib_pi do not display colored overlay for the main file 
named NEurope.grb found at http://om.yr.no/verdata/grib/ 
This file as a 34°lat x 46° lon with a 0.1x0.2° resolution (6'x12') thus with too 
many points. Increasing the 1024x1024 limit allows the display, but once it was done,
I got an offset toward the north of about 4°. (sea screen shots GL_mode1 compared to zygrib):
![gl_mode1](https://cloud.githubusercontent.com/assets/2202442/2741827/a8aba5a0-c6f5-11e3-8126-b711066e653a.jpg)
![zygrib](https://cloud.githubusercontent.com/assets/2202442/2741828/af93e81e-c6f5-11e3-8e4e-bcd4c57e7f40.jpg)
I thought it was due to the file structure, but testing more, I discovered that the offset existed for
all files: An example can be seen in shots Gl_mode2 compared to DC_mode2 for a file of 23° lat and a 0.5x0.5° resolution where I got an offset toward the north of about 1° :
![gl_mode2](https://cloud.githubusercontent.com/assets/2202442/2741833/d064a2f4-c6f5-11e3-8456-233ba3e11512.jpg)
![dc_mode2](https://cloud.githubusercontent.com/assets/2202442/2741834/d727560e-c6f5-11e3-9408-18aa0ef04a74.jpg)
For most files with relatively small zone or low resolution, the offset is not much visible, but it increases according to hight x resolution.
I found that the problem started with the patch named "Improve GRIB Pi memory footprint ..."

I propose this patch which fixes the bug, at least for all the files I have tested.
But I can't test at the moment if the second part of the patch above "...correct wind direction arrows in course-up mode." is still OK even if I think so.
Furthermore, many things are mysterious yet for me in display stuff...
So your validation of the solution is needed. May be there is a better/more efficient solution?
Thanks 
Jean Pierre