Pluggable text-renderer

[katajakasa]

Make the text renderer pluggable, with support for multiple platforms. This idea is slightly extended from the N64 port:

# This replaces the text_settings struct.
typedef struct textbox {
    bool dirty;
    void *userdata;
    
    char *data;
    size_t len;
    
    text_valign valign;
    text_halign halign;
    // ... and all the other options
} textbox;

typedef struct text_engine {
    init_cb init;
    render_cb render; // Called to generate the layout and do the render
    close_cb close; // Called when shutting down the renderer (app exit, renderer change)
} text_engine;

// First, initialize any engine we have. One of these:
// These will set the callback pointers to text_engine.
text_engine engine;
void freetype_engine_init(&engine);
void monospace_engine_init(&engine); // Our old thing

// Then, set it to be used. This would set static variable.
void renderer_engine_set(&engine);

// These replace the old text_settings struct
// Calling any of these sets the dirty = true flag,
void textbox_create(textbox *box);
void textbox_create_with(textbox *box, /* other options for single line call */);
void textbox_from_c(textbox *box, const char *src);
void textbox_from_str(textbox *box, const str *src);
void textbox_set_valign(textbox *box, text_valign valign);

// Calls the text_engine.render(). Render would check if dirty is set, and would do the layout thing
// and set the userdata pointer with cached data (whatever it is). Then just call video renderer with
// whatever needs to be done.
void textbox_render(textbox *box);