Add the TaigaBlend TaigaDrawer
Add screenshot feature to TaigaLayerList.
Rename Taiga.hpp to TaigaCommon.hpp

Soon there will be a new Taiga.hpp which will be used to hold extern
declarations of various game-essential objects.
Move declarations and externs of some game-essential objects

The extern declarations for actors, canvas, key, and mouse have been moved
to the newly-created Taiga.hpp. The actual declarations are now found in
main.cpp
Move several members of TaigaState into global scope.

The members that have been removed from TaigaState are display,
event_queue, frametimer, and ticktimer. This is being done as part
of a larger effort to get rid of TaigaState altogether.
Move TaigaState::quitting to global scope.

It is now named 'taiga_quitting'. See previous commit for details.
Remove some functions from TaigaState.

These functions are TaigaState::quit(), TaigaState::register_event_source()
and TaigaState::retarget_display(), which are no longer needed because
they were merely proxy functions for accessing private data.
Remove the TaigaState* argument from user-defined game functions.

There is no longer anything in TaigaState worth accessing.
Move main game loop to main()

As a result, TaigaState::tick() and TaigaState::run() are no longer
needed nor present.
Move TaigaState::init() to global scope as TaigaInit()
Completely remove the TaigaState class.
Rename TaigaState.*pp to TaigaInit.*pp

Other headers that once included TaigaState.hpp have been updated
accordingly.
Fix comments that refer to the now-nonexistent TaigaState class.