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)