[Fluxus] fbo-pixels merged to master

[gabor papp]

> Did we ever add the switch to turn off clearing the pixel-primitive each
> frame (and retain the existing behaviour)? 
i chose the other method we were talking about. if you just call 
(build-pixels 100 100) or use (load-primitive) a pixel primitive with 
the old behaviour is created, rendering is not activated. there is an 
additional boolean parameter to build-pixels, which activates rendering. 
or you can use (pixels-renderer-activate #t).

> Were there any other things that needed looking at?
i fixed the problem with loading pngs to fbo pixels. copied scott's 
solution from tasks.ss to with-pixels-renderer, so it unbinds the pixel 
primitive if you make a mistake while it is bound.

some minor things i remember we might need to address:
- if you use physics on a pixel primitive, you have to call 
(tick-physics) manually. i don't think it's a big deal.
- there's a slight problem with not power of 2 resolution pixel 
primitives. then the texture is placed in the nearest power of two 
resolution texture. and only a portion of the texture is used for 
rendering. so if you use the pixel primitive as a texture, you have to 
set the texture coordinates manually.
- (hide 1) disables rendering, so you cannot really render to a hidden 
pixel primitive, i usually use (translate) with a huge vector instead of 
(hide), so i move the pixel primitive out of sight.

best,
gabor