This adds support for manipulating the virtual canvas of Images using object accessors (that call underlying C functions MagickImageGetPage and MagickImageSetPage). Both 4-tuple and individual item accessors are added.
It also fixes a previous limitation in the unit tests. The only testable difference between the Merge and Mosaic methods of merging layers is how they handle images with negative page offsets. This feature adds support for testing that difference, so the unit tests have been updated accordingly.