Add possibility to use ImageInterface::fill() with colors or Images

[olivervogel]

Currently ImageInterface::fill() only works with colors. This PR makes it possible to specify images as an alternative. The images can be specified as valid image sources and they are placed over the image as a texture.

Example

$image->fill('images/texture.png', 10, 10);

Issues

GD

• Transparency gets lost in some cases after filling with image texture

Imagick

• Fill with image texture has weird effects on animated images