FIX ImagickBackend backgroundColor

Bugs:
- `Undefined variable: backgroundColor` in ImagickBackend->croppedResize()
- `Unrecognized color string` in  ImagickBackend->paddedResize() - this is because we use a hex color code without a leading hash by default in Image->PaddedImage(). This fix simply prepends a hash char to the hex code if it's not there.

How to reproduce: Run ImagickImageTest