update, start, keypressed, and draw aren't proper callbacks

[tamtaasatiani]

update, start, keypressed, and draw aren't proper callback functions, which causes multiple problems:

• all of these functions have to be defined in main.cpp
• the functions can't be defined multiple times they should be made into callbacks so the function definitions become optional and they can be called from multiple files (which will, for example, make the creation of a timer plugin possible)

[LievenPetersen]

• the functions can't be defined multiple times

For the time being, you could move the main.cpp functions inside their own namespace. That way other files can also define their own update, etc. functions without collision. Those other files could also use their own namespaces, which would avoid the issue even in the current system.

*ahem* Disclaimer: pls note that I'm not very experienced with C++. I hope I'm not misunderstanding namespaces here ^^

[Javier-Lozano]

We can wrap start(), update(), draw() and keypressed(Event) into std::function objects.

At tahm.h

#include <functional>

class Tahm {
...
private:
    std::function<void()> callbackUpdate;
    std::function<void()> callbackDraw;
    std::function<void(Event)> callbackKeypressed;
public:
    void init(std::function<void()> start,
              std::function<void()> update,
              std::function<void()> draw,
              std::function<void(Event)> keypressed);
...
};

At tahm.cpp

void Tahm::init(std::function<void()> start,
                std::function<void()> update,
                std::function<void()> draw,
                std::function<void(Event)> keypressed)
{
    // Store callbacks
    callbackUpdate = update;
    callbackDraw = draw;
    callbackKeypressed = keypressed;

    start(); // We could call start right away
    window->init();
    renderer->init();
    audio->setupDevice();
}

void Tahm::run() // We can now move the main loop inside the class
{
    handleEvents(); // callbackKeypressed runs inside here
    callbackUpdate();
    renderer->prepare();
    callbackDraw();
    renderer->present();
}