Manually set extents on layers

[randomjunk]

Add an option to manually set the extent on a layer instead of requiring the datasource to find it.

For example: postgis inputs have a very unreliable estimate_extent option -- if you have to turn this off then postgis does a table scan to discover the extent, which can be very inefficient, especially if you know the datasource entirely covers the area you are rendering. It would be better to specify the extent manually in this case.

[artem]

use 'extent' parameter to specify user defined extent for a layer (XML) :

<Parameter name="extent">-20037508.3428,-19929239.1134,20037508.3428,19929239.1134</Parameter>

or in Python

ds = PostGIS(........, extent="-180,-90, 180 ,90")