Porting the postprocessing code

Porting the posteffect code from Crystal Space soc2011 branch to the newest(soc2013/postprocessing), wasn't too hard as I expected.
In the porting process I learned a bit more on how the current postprocessing code work, I found the critical parts to start the work and, of course, I found bugs too.

• hdr is buggy, something related to frame buffers deallocation
• Basic posteffect chaining is working
• Removing/adding the effcts is buggy, I think it's due to the PostEffectsSupport not chaining the rendertargets correctly
• The posteffect code doesn't yet reflect a design focused in the use of multiples effects

There are some minor bugs too, like wrongly offseted texture coordinates and pixelSize shader variable not setup correctly, for these I will commit fixes soon.

I also did a small demo to test posteffect chaining. Here is an image showing three simple posteffects chained

The next step in the work is to start improving the desing of the posteffect code to reflect the needs of the proposed effects.