added sliding window for large image inference

[aspaul20]

PaddleOCR does not work on large documents/images, this feature consists of a sliding window inference method, which although takes longer (expectedly), uses a sliding window to create slices of the input image and run detection+recognition on it. Unlike the default code, it gives correct results. The vertical and horizontal strides are adjustable by the user.

Output on an image of dimensions (5088x3600):

Without sliding window:

With sliding window:

Note: It could use a postprocessing step where the adjacent detections are merged into one, if needed.