Update colour_t to use a union

include/window/window.h

@@ -7,10 +7,17 @@
 #include <stdbool.h>
 
 typedef struct {
-  u8 r;
-  u8 g;
-  u8 b;
   u8 a;
+  u8 b;
+  u8 g;
+  u8 r;
+} rgba_t;
+
+typedef struct {
+  union {
+    rgba_t rgba;
+    u32 colour;
+  };
 } colour_t;
 
 typedef struct {