cogl/matrix: Add cogl_matrix_get_value

This will allow us stop accessing members of the structure
directly. Also mark all struct members as private.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1439

cogl/cogl/cogl-matrix.c

@@ -1941,6 +1941,14 @@ cogl_matrix_get_array (const CoglMatrix *matrix)
   return (float *)matrix;
 }
 
+float
+cogl_matrix_get_value (const CoglMatrix *matrix,
+                       unsigned int      row,
+                       unsigned int      column)
+{
+  return MAT ((float *)matrix, row, column);
+}
+
 void
 cogl_matrix_transform_point (const CoglMatrix *matrix,
                              float *x,

cogl/cogl/cogl-matrix.h

@@ -86,6 +86,8 @@ G_BEGIN_DECLS
  */
 struct _CoglMatrix
 {
+  /*< private >*/
+
   /* column 0 */
   float xx;
   float yx;
@@ -110,8 +112,6 @@ struct _CoglMatrix
   float zw;
   float ww;
 
-  /*< private >*/
-
   /* Note: we may want to extend this later with private flags
    * and a cache of the inverse transform matrix. */
   float          COGL_PRIVATE (inv)[16];
@@ -491,6 +491,17 @@ cogl_matrix_init_from_matrix (CoglMatrix       *matrix,
 COGL_EXPORT const float *
 cogl_matrix_get_array (const CoglMatrix *matrix);
 
+/**
+ * cogl_matrix_get_value:
+ * @matrix: A 4x4 transformation matrix
+ *
+ *
+ */
+COGL_EXPORT float
+cogl_matrix_get_value (const CoglMatrix *matrix,
+                       unsigned int      row,
+                       unsigned int      column);
+
 /**
  * cogl_matrix_init_from_euler:
  * @matrix: A 4x4 transformation matrix