Ensure thread safe handling for the drive-on-left/right flag

The drive on left flag is set via a static method. This was not thread safe when using more than one thread. The flag is now stored as ThreadLocal variable which ensures that the value is used only in the current thread.

git-svn-id: http://svn.mkgmap.org.uk/mkgmap/trunk@2670 25d90789-57f7-4ee0-8453-03a3dfeeeb22
Do not always set drive-on-left flag to false if option precomp-sea is set

In case a tile has mixed sea/land content and uses the precomp-sea option the precompiled sea tiles are loaded by an OsmMapDataSources instance. This creates a an option less StyledConverter which sets the drive-on-left flag to false. This overrides the drive-on-left/right option of the config file. 
This change adds an internal option "ignore-drive-on-left" which does not initialize the drive-on-left flag in the StyleConverter. This option is only set by the SeaGenerator when loading precompiled sea tiles.

git-svn-id: http://svn.mkgmap.org.uk/mkgmap/trunk@2671 25d90789-57f7-4ee0-8453-03a3dfeeeb22