[FEATURE] Opening large images takes a long time

[mherzog01]

Our images are 35 MB BMP (e.g. sample bitmap). Opening them in the current version of the software on my Windows laptop (no GPU) takes 15 sec or more.

I believe the issue is the call to image_pil.save(f, format=format) in label_file.py. The current code converts the .bmp to PNG, which appears to take a long time. If we just did the save to the same format (.bmp saves to BMP), performance is decent. See below for a possible solution.

    @staticmethod
    def load_image_file(filename):
        try:
            image_pil = PIL.Image.open(filename)
        except IOError:
            logger.error("Failed opening image file: {}".format(filename))
            return

        # apply orientation to image according to exif
        image_pil = utils.apply_exif_orientation(image_pil)

        with io.BytesIO() as f:
            ext = osp.splitext(filename)[1].lower()
            if PY2 and QT4:
                format = "PNG"
            elif ext in [".jpg", ".jpeg"]:
                format = "JPEG"
            elif ext == ".bmp":        # <==== Modified
                format = "BMP"        # <==== Modified
            else:
                format = "PNG"
            image_pil.save(f, format=format)
            f.seek(0)
            return f.read()