generic API for widget rendering and styling;  see: doc/ref/style.html


/* A stroke is typically a line or arc drawn with a pen that has a color
   and a width. The pen is often square-tip and width is interpreted as
   side length of the square. Tip shape and interpretation of width is
   defined by the style implementation. */
typedef struct mbtk_stroke_s {
  mbtk_color_t color;
  unsigned char width;
} mbtk_stroke_t;

/* A style implemenetation */
struct mbtk_style_s {
  /* Set a property of a style struct */
  int (*getset_prop)(const mbtk_style_t *style, mbtk_styleprop_t *dst, mbtk_kw_t key, mbtk_arg_t *get, mbtk_arg_t *set);

  /* Return 0 and set the minimum size for rendering the specified widget; when
     returns -1, a widget-specific minsize funciton will need to be called instead */
  int (*minsize)(const mbtk_style_t *style, mbtk_widget_t *w, long *mx, long *my);

  /* called to draw the base or overlay widget; first all bases are drawn
     then all overlays, recursively, from the root down in a BFS. If returns
     non-zero, the style has no idea how to draw the widget and the fallback
     mechanism should be invoked */
  int (*draw_widget)(const mbtk_style_t *style, mbtk_widget_t *w);
  int (*draw_widget_overlay)(const mbtk_style_t *style, mbtk_widget_t *w);

  /* Low level draw functions: background rectangle and box frame; used by
     widget fallback draw functions */
  void (*draw_box_frame)(mbtk_window_t *win, const mbtk_styleprop_t *sp, mbtk_widget_flags_t flg, long x, long y, long w, long h);
  void (*draw_box_overlay)(mbtk_window_t *win, const mbtk_styleprop_t *sp, mbtk_widget_flags_t flg, long x, long y, long w, long h);
  void (*draw_bg_rect)(mbtk_window_t *win, const mbtk_styleprop_t *sp, mbtk_widget_flags_t flg, long x, long y, long w, long h);
  void (*set_fg_color)(mbtk_window_t *win, const mbtk_styleprop_t *sp_, mbtk_widget_flags_t flg);
  void (*min_box_frame_size)(mbtk_window_t *win, const mbtk_styleprop_t *sp_, mbtk_widget_flags_t flg, long *left, long *right, long *top, long *bottom);

  long propsize; /* size of the opaque styleprop struct, in bytes */
};

/* Standard style prop field names */
extern mbtk_kw_t mbtk_kw_bg_color, mbtk_kw_bg_activated_color, mbtk_kw_fg_color;
extern mbtk_kw_t mbtk_kw_hilight_color;
extern mbtk_kw_t mbtk_kw_frame_width, mbtk_kw_text_halign, mbtk_kw_text_valign;


/* Get or set style property. Return 0 on success. */
int mbtk_display_style_get_prop(mbtk_display_t *src, mbtk_kw_t key, mbtk_arg_t *dst);
int mbtk_display_style_set_prop(mbtk_display_t *dst, mbtk_kw_t key, mbtk_arg_t value);
int mbtk_style_get_prop(const mbtk_style_t *style, mbtk_styleprop_t *src, mbtk_kw_t key, mbtk_arg_t *dst);
int mbtk_style_set_prop(const mbtk_style_t *style, mbtk_styleprop_t *dst, mbtk_kw_t key, mbtk_arg_t value);