Added some helpful log() to Render2D.
Added support for preprocessing resources to the resource management.
Resources defined in the resource index can now be specified in a
<preprocess> tag and are then processed automatically with their
dedicated resource processor. This allows one to define resources that
should/need to be processed without the need to change code.

Currently the processing is done before the screen manager is started.
Optimally this should take place right aftrer loading but Stage3D is not
ready early enough for that and texture resources require Stage3D to be
initialized to process them.