That's how it should be done, because the SDL headers aren't going to be
installed in this repository. The game was a bit inconsistent before but
now it isn't anymore.
Previously, it was used in order to clear a block and deactivate it, and
the constructor function simply called clear() in order to not duplicate
code. However, clear() is no longer necessary (just remove the block
from the blocks vector), and so we can put initialization right back in
the constructor function.
This removes the variables obj.nblocks, as well as removing the 'active'
attribute from the block object. Now every block is guaranteed to be
real without having to check the 'active' variable.
Removing a block while iterating now uses the removeblock_iter() macro.
