Kwimage

Added
* New function: `exactly_1channel`.

Fixed
* kwimage.imresize now works when the input image has dsize=(1, 1)

Changed
* Small speedups
* Add cv2 backend to `fourier_mask`, which is a 3x speedup, and set this to the default.

Changed
* imresize now returns the original image if all scale / dsize arguments are unspecified or None.
* Polygon.fill can now work in non-inplace cases, but the `assert_inplace` flag must be set to 0.
* Allow nan as a nodata_method as alias for float

Fixed
* `labels` in `draw_on` now interprets integers as booleans
* Fixed shapely warning about the "type" attribute. Now using `geom_type` instead.

Fixed

* imresize now accepts input dtypes: bool, int32
* imresize now outputs the same channel dimensions as given in the input
* Polygon.union is fixed. (previously it did intersection)
* replaced np.bool with bool
* Fixed issue when drawing text on an unspecified image with an rgba background color

Changed

* Better shapely and gdal transform support in Affine
* Improved agreement between polygon and box draw_on methods.
* Added Color.adjust
* Moved color data to its own module
* `fill_nans_with_checkers` uses a muted checkerboard which is less visually jarring

Fixed

* Fixed issue in kwimage.Box where some functions (e.g. `tl_x`) were returning
incorrectly shaped.
* Bug in nms with cython cpu.