AMF: instance subelements don't respect specified float64 format/precision

[HuguesDelorme]

When exporting AMF contents the deltax, deltay, .... subelements values don't respect the float64 precision indicated in gmio_amf_write_options::float64_prec

Example :
float64_prec = 4

<?xml version="1.0" encoding="UTF-8"?>
<amf unit="millimeter" version="1.2">
<material id="0">
<color><r>1.0000</r><g>1.0000</g><b>1.0000</b><a>0.0000</a></color>
</material>
<material id="1">
<color><r>1.0000</r><g>0.0000</g><b>0.0000</b><a>0.0000</a></color>
</material>
...
<constellation id="0">
<instance objectid="0"><deltax>175.0000000000000</deltax><deltay>67.50000000000000</deltay><deltaz>70.00000000000000</deltaz><rx>0.000000000000000</rx><ry>90.00000000000000</ry><rz>0.000000000000000</rz></instance>
<instance objectid="0"><deltax>2.000000000000000</deltax><deltay>67.50000000000000</deltay><deltaz>70.00000000000000</deltaz><rx>0.000000000000000</rx><ry>90.00000000000000</ry><rz>0.000000000000000</rz></instance>
...

Early investigation shows this is caused by the call to gmio_ostringstream_write_xmlelt_f64() which internally passes NULL to gmio_ostringstream_write_f64().