Code review comment for lp:~nha/widelands/opengl

1. I used the surface cache directly. I am a little unhappy that we copy and paste the image loading code here. I added a TODO for this which will let me sleep :-)

2. I do not understand this argument. const Klass* and const Klass& reads clearer to me. And you always read right to left. Oh well - mainly bikeshedding. I settled this for now by adding a new style checker rule. It reports > 1000 violations right now oO. Will see if I can get them fixed tonight.

3. done.
4. Couldn't be bothered for now to touch this. Just add as much const in as you can in the future. It makes life easier often (I just learned this at work basically).
5. Left as is. I trust you that it is not on timing critical paths.
6. done.

I did the fixes in the common branch and merged it into trunk. How cool is that: Widelands became faster, less demanding on the hardware and uses less memory and these improvements all came in one commit.