upload_creator: ask ImageMagick to use internal SVG code to determine size (PR #13783)

This change largely targets dev users, but it could potentially change behaviour in production.

Jamie Wilson & I debugged a problem where “should not be larger than the maximum thumbnail size” would fail due to timeouts.

On our systems, on ImageMagick 7.1.0-2, with inkscape installed, IM would attempt to rasterise the svg then check the resulting filesize, causing the test to timeout.

As of now, we haven’t found a way to cause this to behave better, but have a workaround in that forcing IM to use the internal renderer (MSVG:) would perform quickly, but error out on more complex files. We can at least try the fast path before falling back to the potentially slower method.

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