add a bit of documentation

namespace plotfx {

/**

 * The layer is the central rendering context on which 2D vector graphics

 * operations, such as rendering text and drawing polygons, take place.

 *

 * The Layer struct itself is an abstract, transparent handle. It consists mostly

 * of function pointers that have to be bound to some specific rendering backend

 * for the Layer to be useful.

 *

 * Users should create layers using one of the layer_bind_* methods, for example

 * `layer_bind_png` or `layer_bind_svg`.

 *

 * Since the layer is a low-level interface, it is recommended that you use one

 * of the many drawing helper functions, such as `strokePath` or `drawText`, instead

 * of interacting directly with the Layer. Nevertheless, all supported drawing

 * operation are available through the `apply` method of the layer. Please refer

 * to `layer_op.h` for more information on the low-level operations API.

 *

 * Once you are done with all drawing calls, you can retrieve the result image by

 * calling the `data` function.

 */

struct Layer {

  const double width;

  const double height;