Fix duplicate clip-path attribute and restore piped stdin input

[turbinefish]

Two independent fixes found while converting EMF figures extracted from real Office documents. Both are reproducible on current master and on the 1.8.1 tag.

1. endFormDraw() emits a second clip-path attribute

startPathDraw() already writes clip-path when it opens the element.
endFormDraw() wrote it a second time on the same element, so any record that goes through both (the pie/chord/arc family) produced:

    <path clip-path="url(#clip-N)" ... clip-path="url(#clip-N)" />

That is not well-formed XML (duplicate attribute), so a conforming SVG renderer rejects the whole document. One such record makes the entire figure unrenderable, not merely mis-drawn:

    $ emf2svg-conv -i chart.emf -o chart.svg && rsvg-convert -o chart.png chart.svg
    Error reading SVG chart.svg: XML parse error: Attribute clip-path redefined

Dropping the duplicated call in endFormDraw() is enough; the attribute is still emitted once, by startPathDraw().

2. emf2svg-conv can no longer read a pipe

The converter seeks to the end of the input to size its buffer before reading.
That works for a regular file but not for a pipe, where the reported size is 0,so the tool aborts (std::bad_alloc) on:

    $ cat figure.emf | emf2svg-conv -i /dev/stdin -o /dev/stdout

Reading the stream sequentially restores the pre-1.8 behaviour. Ownership of the buffer is unchanged, so the rest of main() is untouched.

Verification

Tested on 12 EMF files extracted from real-world Office documents (a Word design
document and a legacy PowerPoint deck), all of them dual EMF/EMF+ files:

• Before: 11/12 converted, 1/12 was rejected outright by the renderer.
• After: 12/12 convert and render, and the 11 previously working outputs are byte-for-byte identical — so the first change only affects the records that were producing invalid markup.
• The pipe path was verified end to end with cat f.emf | emf2svg-conv -i /dev/stdin -o /dev/stdout | rsvg-convert -b white.

Happy to split this into two pull requests if you prefer them separate.