Add a vec2s struct type for consistency

There's a vec2 type, so there should probably be a struct version of it
too. Even if no functions use it right now, if a library user (like me)
needs a 2-element vector, they don't need to roll their own.

include/cglm/types-struct.h

@@ -10,6 +10,16 @@
 
 #include "types.h"
 
+typedef union vec2s {
+#ifndef CGLM_NO_ANONYMOUS_STRUCT
+  struct {
+    float x;
+    float y;
+  };
+#endif
+  vec2 raw;
+} vec2s;
+
 typedef union vec3s {
 #ifndef CGLM_NO_ANONYMOUS_STRUCT
   struct {