DXF: Handle linetype scaling

[Alan Thomas]

Currently the DXF driver does not handle linetype scaling at all. Linetypes may be scaled by the scale factor specified on the entity (group code 48) or in the global $LTSCALE variable.

This patch implements linetype scaling in the reading-in side of the DXF driver.

I haven't touched the writer, and it will need a lot of reworking if it is to handle linetype scaling properly. The incorrect round-tripping of DXF files with linetype scales is not new, but I have added a comment to drv_dxf.html to make users aware of this caveat.

[Alan Thomas]

In 40630:

DXF: Handle linetype scales (fixes #7129)

[Even Rouault]

In 40633:

DXF: use prefixed increment operator to please ubuntu 16.04 cppcheck (refs #7129)