Change const accessors to return const temporary values, so that
	e.g. image[p] = v; fails to compile when image is const.

	* src/core/pred.hh (operator[] const): Return a const value.
	* src/core/internal/window.hh (operator[] const): Likewise.
	* src/core/internal/w_window.hh (operator[] const): Likewise.
	* src/core/internal/image1d.hh (size const, operator[] const,
	operator() const): Likewise.
	* src/core/internal/image2d.hh (size const, operator[] const,
	operator() const): Likewise.
	* src/core/internal/image3d.hh (size const, operator[] const,
	operator() const): Likewise.

	* src/core/internal/image1d_data.hh,
	src/core/internal/image2d_data.hh,
	src/core/internal/image3d_data.hh: More comments.

    prcs2git-id: 0_3.11