Feature #8725-revival: Fast rendering of geometries, fix bug slow rendering in ubuntu and other minor changes.

Implements fast rendering of LineStrings and Polygons simplificating the geometries before render them in QGIS. Also disable 'Antialiasing' when it is possible.

This PR is inspired in merged PR #980, but it contains several changes based on advice received ( @matthias-kuhn thanks! ) and new features:

+ About code, the simplification is configured in a new QgsSimplifyMethod class which indicates how to simplify the geometries in a feature iterator.
+ Now, the user can define where the simplification executes (There is a new option in settings panel), locally after fetch the geometry from provider, or simplifying it on provider side. e.g. In postgres provider, first option simplifies the geometry already fetched locally, but the second option simplifies the geometry in database using the function ST_Simplify.
+ The settings panel also shows the simplification threshold in pixel units as @timlinux suggested me.

About postgres simplification, the ST_simplify function needs a tolerance parameter, I use map2pixel/5.0 as input value, it is experimental and I must define it better (All ideas are welcome). This simplification can be applicable to other database providers (MySQL, SQL Server, Oracle...)