Bug fix for the Ganesha Statue scene (see issue #135).

Add some command line options to replace the '--cropwindow' option (of the C++ counterpart):

$ ./target/release/rs_pbrt -h
rs_pbrt 0.9.2
Parse a PBRT scene file (extension .pbrt) and render it

USAGE:
    rs_pbrt [OPTIONS] <path>

FLAGS:
    -h, --help       Prints help information
    -V, --version    Prints version information

OPTIONS:
        --cropx0 <cropx0>        Specify an image crop window <x0 x1 y0 y1> [default: 0.0]
        --cropx1 <cropx1>        Specify an image crop window <x0 x1 y0 y1> [default: 1.0]
        --cropy0 <cropy0>        Specify an image crop window <x0 x1 y0 y1> [default: 0.0]
        --cropy1 <cropy1>        Specify an image crop window <x0 x1 y0 y1> [default: 1.0]
    -t, --nthreads <nthreads>    use specified number of threads for rendering [default: 0]

ARGS:
    <path>    The path to the file to read