GPF Tile API to provide method to directly check sample validity

There should be an easier way for developers to check the availability of a tile's sample value at a given image position, e.g. add to the Tile interface:

/**
 * Check if a sample value is valid at a given image pixel position.
 * @param x the image pixel x-coordinate
 * @param y the image pixel y-coordinate
 * @return true, if the sample is valid
 */
boolean isSampleValid(int x, int y);