Union layer new processing options

[tamas]

We'll require to add 3 new processing options for the union layer.

UNION_STATUS_CHECK (TRUE or FALSE): Controls whether the status of the source layes should be checked and the invisible layers (STATUS=OFF) should be skipped. Default value is FALSE;

UNION_SCALE_CHECK (TRUE or FALSE): Controls whether the scale range of the source layes should be checked and the invisible layers (falling outside of the scale range and zoom range) should be skipped. Default value is TRUE;

UNION_SRCLAYER_CLOSE_CONNECTION: Override the connection pool setting of the source layers. By introducing this setting we alter the current behaviour which is equivalent to:

UNION_SRCLAYER_CLOSE_CONNECTION=ALWAYS

[tamas]

Changes applied in trunk (r11774)

[tamas]

Documentation added in r12689