mesa: add AllocTextureImageBuffer() driver hook

src/mesa/main/dd.h

@@ -480,6 +480,12 @@ struct dd_function_table {
    void (*DeleteTextureImage)(struct gl_context *ctx,
                               struct gl_texture_image *);
 
+   /** Called to allocate memory for a single texture image */
+   GLboolean (*AllocTextureImageBuffer)(struct gl_context *ctx,
+                                        struct gl_texture_image *texImage,
+                                        gl_format format, GLsizei width,
+                                        GLsizei height, GLsizei depth);
+
    /** 
     * Called to free tImage->Data.
     */