Geometry Nodes: Use factor slider for distribution density factor

Though the factor isn't so useful to adjust by itself, and is mostly
useful when used with a field connected, the slider from 0 to 1 can
help to make it clear that it's just used as a multiplier for the max
density after distribution.

Differential Revision: https://developer.blender.org/D12654

source/blender/nodes/geometry/nodes/node_geo_distribute_points_on_faces.cc

@@ -50,6 +50,7 @@ static void geo_node_point_distribute_points_on_faces_declare(NodeDeclarationBui
       .default_value(1.0f)
       .min(0.0f)
       .max(1.0f)
+      .subtype(PROP_FACTOR)
       .supports_field();
   b.add_input<decl::Int>("Seed");
   b.add_input<decl::Bool>("Selection").default_value(true).hide_value().supports_field();