Large backtrack-stages setting causes a crash inside `ImagePyramid::build_gaussian`

[toomuchsalt]

Setting backtrack-stages to more than log2(image-dimension) leads to a panic inside ImagePyramid::build_gaussian

To Reproduce
Steps to reproduce the behavior:
cargo run -- --backtrack-stages 10 -o out.png generate 500x500.png

Device:

• OS: Linux 5.3.0-13
• Compiler: rustc 1.38.0-nightly

Additional context
Crash backtrace

   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.34/src/backtrace/libunwind.rs:88
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.34/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:47
   3: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:36
   4: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:200
   5: std::panicking::default_hook
             at src/libstd/panicking.rs:214
   6: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:477
   7: std::panicking::continue_panic_fmt
             at src/libstd/panicking.rs:384
   8: rust_begin_unwind
             at src/libstd/panicking.rs:311
   9: core::panicking::panic_fmt
             at src/libcore/panicking.rs:85
  10: core::panicking::panic
             at src/libcore/panicking.rs:49
  11: core::option::Option<T>::unwrap
             at /rustc/60960a260f7b5c695fd0717311d72ce62dd4eb43/src/libcore/macros.rs:12
  12: image::imageops::sample::horizontal_sample
             at /`hidden`/.cargo/registry/src/github.com-1ecc6299db9ec823/image-0.22.3/./src/imageops/sample.rs:199
  13: image::imageops::sample::resize
             at /`hidden`/.cargo/registry/src/github.com-1ecc6299db9ec823/image-0.22.3/./src/imageops/sample.rs:681
  14: texture_synthesis::img_pyramid::ImagePyramid::build_gaussian
             at lib/src/img_pyramid.rs:29
  15: texture_synthesis::img_pyramid::ImagePyramid::new
             at lib/src/img_pyramid.rs:16
  16: texture_synthesis::Example::resolve
             at lib/src/lib.rs:375
  17: texture_synthesis::SessionBuilder::build
             at lib/src/lib.rs:675
  18: texture_synthesis::real_main
             at cli/src/main.rs:283
  19: texture_synthesis::main
             at cli/src/main.rs:182
  20: std::rt::lang_start::{{closure}}
             at /rustc/60960a260f7b5c695fd0717311d72ce62dd4eb43/src/libstd/rt.rs:64
  21: std::rt::lang_start_internal::{{closure}}
             at src/libstd/rt.rs:49
  22: std::panicking::try::do_call
             at src/libstd/panicking.rs:296
  23: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:80
  24: std::panicking::try
             at src/libstd/panicking.rs:275
  25: std::panic::catch_unwind
             at src/libstd/panic.rs:394
  26: std::rt::lang_start_internal
             at src/libstd/rt.rs:48
  27: std::rt::lang_start
             at /rustc/60960a260f7b5c695fd0717311d72ce62dd4eb43/src/libstd/rt.rs:64
  28: main
  29: __libc_start_main
  30: _start
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

Process finished with exit code 101

[Jake-Shadle]

Thanks for reporting this, will need to add some guards to ensure that parameter stays in a proper range based on inputs.