I noticed that $scale and @map_scale exist in parallel, but are not
consistently behaving between QGIS Desktop and QGIS server.
$scale: works fine on both Desktop and Server, but expression preview in
QGIS Desktop fails.
@map_scale: works fine on QGIS Desktop and also in expression preview,
but fails on QGIS Server.
Context: expression to calculate letter spacing in a label for different
map scale. Tested on QGIS Master 2.x on Windows 7 64bit.
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