Let the templates of smoke sources set how fast their particles rise or fall

shaders/particle_common.glsl

@@ -22,6 +22,7 @@ struct smoke_source_s
 	vec4 sizechange; // x: per second, y: min, z: max
 	vec4 opacitychange; // x: per second, y: min, z: max
 	vec4 motion; // x: rise, y: damping (per second, of the vertical velocity), z: least vertical velocity
+	vec4 settling; // x: velocity the particles settle at, m/s upwards, y: drag, per second, z: 1 when they settle (in place of motion)
 	vec4 color; // rgb: colour of the smoke, lit by the lights of the pass
 	vec4 oldcolor; // rgb: colour lit as the cpu does it, without the lights (gfx.smoke.lighting off)
 };