Feature: Random marker fill symbol layer type

This commit adds a new fill symbol layer type "Random marker fill". It
allows polygon features to be rendered using a marker symbol placed
at random locations within the polygon boundary.
Options include:

• number of marker symbols to render for the polygon
• whether markers rendered near the edges of polygons should be clipped to the polygon boundary or not
• an optional random number seed, to give consistent placement of markers whenever maps are refreshed (also allows random placement to play nice with QGIS server and tile-based rendering)

This feature was funded by SLYR

This feature was developed by Nyall Dawson (North Road)