lv_color.h fix uninitialized alpha chanel with 24 bit color

lv_misc/lv_color.h

@@ -248,6 +248,10 @@ static inline lv_color_t lv_color_mix(lv_color_t c1, lv_color_t c2, uint8_t mix)
     ret.red =   (uint16_t)((uint16_t) c1.red * mix + (c2.red * (255 - mix))) >> 8;
     ret.green = (uint16_t)((uint16_t) c1.green * mix + (c2.green * (255 - mix))) >> 8;
     ret.blue =  (uint16_t)((uint16_t) c1.blue * mix + (c2.blue * (255 - mix))) >> 8;
+#if LV_COLOR_DEPTH == 24
+    ret.alpha = 0xFF;
+#endif
+
     return ret;
 }