explicit border in stream table about pdf2docx

[dothinking]

Current solution: based on the the Border data structure for #41, finalize border with explicit borders.

Taking h-border, as shown in the screenshot, for example:

• finalize h-border with y-coordinate of the explicit border if passing through its valid range
• finalize boundary borders, i.e. left & right, with the x0 and x1 coordinates of explicit border if x0, x1 are in the valid range

[dothinking]

Limitation: the idea is to update stream borders (parsed from blocks layout) with explicit borders, but the parsed stream borders may be not compatible with explicit borders.