zephr/game: various fixes and improvements

- zephr: handle newlines in text
- fix(zephr): don't overflow color channels bytes when multiplying
  colors
- fix(zephr): don't fallback to UI_CONSTRAINT_FIXED when setting the x
  or y constraint to UI_CONSTRAINT_RELATIVE_PIXELS
- game: added how to play dialog
- game: ability to start a new game

game.h

@@ -13,6 +13,7 @@ typedef enum MoveDir {
 typedef enum IconTexture {
     HELP_ICON,
     SETTINGS_ICON,
+    CLOSE_ICON,
 
     ICON_TEXTURE_COUNT,
 } IconTexture;
@@ -37,6 +38,7 @@ typedef struct Game {
 
     TextureId icon_textures[ICON_TEXTURE_COUNT];
     bool quit_dialog;
+    bool help_dialog;
 } Game;
 
 void draw_board(void);