Take MltInput out of the EffectChain.

Having the MltInput be an Input which forwards down to the real implementation
has been a source of multiple headaches, and now lastly, when finalize()
disappeared, source of a broken build. We still need the unified
set_pixel_pointer() etc., but the class is now simply a holder of the Input*,
not a forwarder as viewed from the EffectChain.
Remove the FBO freelist.

FBOs are cheap to construct and delete (they carry almost no state),
so it is less complex just to do it on the fly. It also gives less
leakage, as we use new contexts all the time.
Remove the glsl_texture freelist.

Given that new/delete on such small objects are cheap and this happens
rarely, it is probably not worth the extra complexity. (In the process,
fix a minor bug related to out-of-memory; not that new will actually
ever return NULL on any compilers newer then MSVC6.)