Enable remotecv to process ICO files

[RaphaelVRossi]

When Remotecv process some ICO file, it will fail with this Opencv error:

EXCEPTION(most recent call first)
error: OpenCV(4.7.0) /io/opencv/modules/imgproc/src/color.cpp:182: error: (-215:Assertion failed) !_src.empty() in function 'cvtColor'

  File "pyres/worker.py", line 256, in process
    return job.perform()
  File "pyres/job.py", line 83, in perform
    return payload_class.perform(*args)
  File "remotecv/pyres_tasks.py", line 17, in perform
    points = cls.processor.detect(detection_type, image_data)
  File "remotecv/image_processor.py", line 29, in detect
    return self.run_detections(image, detector.split("+"))
  File "remotecv/image_processor.py", line 36, in run_detections
    points = self.detectors[detector].detect(image)
  File "remotecv/detectors/face_detector/__init__.py", line 24, in detect
    features = self.get_grayscale_equalized_features(image)
  File "remotecv/detectors/__init__.py", line 60, in get_grayscale_equalized_features
    img = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)

This PR will change the format from ICO to PNG, in order to avoid this error.

ICO to test:
https://raw.githubusercontent.com/thumbor/remotecv/enable-process-ico-files/tests/fixtures/one_face.ico

[codeclimate]

Code Climate has analyzed commit ecd739b and detected 0 issues on this pull request.

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[coveralls]

Pull Request Test Coverage Report for Build 4198464283

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 84.852%

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Totals
Change from base Build 3883074110:	0.0%
Covered Lines:	414
Relevant Lines:	519

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💛 - Coveralls

[devppjr]

LGTM 🚀