ST_RemoveIrrelevantPointsForView() - Make optimizations based on cartesian math optional

[gluser1357]

The latest optimizations of ST_RemoveIrrelevantPointsForView() (see changes in ​https://github.com/postgis/postgis/pull/767) for reducing the resulting point number make use of cartesian math and assume that coordinates won't be reprojected after applying ST_RemoveIrrelevantPointsForView() and before rendering.

This is fine for common use cases where no reprojection is taking place, but for other use cases (where coordinates are stored e. g. as lat/lon values, get pre-processed with ST_RemoveIrrelevantPointsForView(), reprojected and rendered), rendering artifacts might occur. For such use cases it would make sense to turn off those optimizations that use cartesian math. This can be achieved by using a new flag cartesian_hint. By default (if not specified), no optimizations are done.

I'll post a link to an appropriate PR here soon.