Add compile time checks for size of public structs

To help catch accidental changes to the size of public structs that can
be allocated on the stack this patch adds compile time checks that our
struct sizes haven't changed.

cogl/cogl-euler.h

@@ -157,6 +157,7 @@ struct _CoglEuler
   float padding3;
   float padding4;
 };
+COGL_STRUCT_SIZE_ASSERT (CoglEuler, 32);
 
 /**
  * cogl_euler_init:

cogl/cogl-matrix.h

@@ -108,6 +108,7 @@ struct _CoglMatrix
   unsigned long  COGL_PRIVATE (flags);
   unsigned long  COGL_PRIVATE (_padding3);
 };
+COGL_STRUCT_SIZE_ASSERT (CoglMatrix, 128 + sizeof (unsigned long) * 3);
 
 /**
  * cogl_matrix_init_identity:

cogl/cogl-quaternion.h

@@ -132,6 +132,7 @@ struct _CoglQuaternion
   float padding2;
   float padding3;
 };
+COGL_STRUCT_SIZE_ASSERT (CoglQuaternion, 32);
 
 /**
  * cogl_quaternion_init:

cogl/cogl-types.h

@@ -47,6 +47,16 @@ G_BEGIN_DECLS
 #define COGL_PRIVATE(x) private_member_ ## x
 #endif
 
+/* To help catch accidental changes to public structs that should
+ * be stack allocated we use this macro to compile time assert that
+ * a struct size is as expected.
+ */
+#define COGL_STRUCT_SIZE_ASSERT(TYPE, SIZE) \
+typedef struct { \
+          char compile_time_assert_ ## TYPE ## _size[ \
+              (sizeof (TYPE) == (SIZE)) ? 1 : -1]; \
+        } _ ## TYPE ## SizeCheck
+
 /**
  * CoglHandle:
  *
@@ -344,6 +354,7 @@ struct _CoglColor
   guint32 COGL_PRIVATE (padding1);
   guint32 COGL_PRIVATE (padding2);
 };
+COGL_STRUCT_SIZE_ASSERT (CoglColor, 16);
 
 /**
  * CoglTextureVertex:
@@ -364,6 +375,7 @@ struct _CoglTextureVertex
 
   CoglColor color;
 };
+COGL_STRUCT_SIZE_ASSERT (CoglTextureVertex, 36);
 
 /**
  * CoglTextureFlags: