partition_pptx: Sort shapes by position (left to right and top to bottom) before iterating shapes

[Bill-XU]

Hello,

Unstructured is a great tool for extracting texts from PPTXs.
But the extracted texts are not somehow in human reading order.

It seems that unstructured relies on layering order of the shapes.
I think this is for some performance concerns,
but is it possible to change this behavior when reading order should be considered for some PPTXs.

Best regards,
Bill