The C++ importer incorrectly treated the contents of all TEXT and MTEXT
objects as beind encoded as UTF-8, but this is not true for DXF files
before AutoCAD 2007, where the encoding is "plain ASCII" plus some in-band
\U+dddd encoding. This would cause errors if the text contained non-ASCII
characters such as the Degree Sign.
This change causes the correct encoding to be used.
If the MTEXT exceeds 250 characters, the contents are broken up into
several records in the DXF file. The C++ importer has been changed
to collect all these pieces into a single string containing all the text.
