Change particle draw_size default and limits

Particle draw size can now no longer be less than 1px, and the default
value has been changed to 4.

source/blender/blenkernel/intern/particle.c

@@ -3289,6 +3289,7 @@ static void default_particle_settings(ParticleSettings *part)
 
 	part->omat = 1;
 	part->use_modifier_stack = false;
+	part->draw_size = 4;
 }