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

text.h

@@ -29,6 +29,13 @@ typedef struct GlyphInstanceList {
   int capacity;
 } GlyphInstanceList;
 
+// TODO: impl this to align text relative to its box
+typedef enum TextAlign {
+  TEXT_ALIGN_LEFT,
+  TEXT_ALIGN_CENTER,
+  TEXT_ALIGN_RIGHT
+} TextAlign;
+
 void new_glyph_instance_list(GlyphInstanceList *list, u32 capacity);
 int init_fonts(const char *font_path);
 Sizef calculate_text_size(const char *text, int font_size);