shape: Use an explicit move_to to set rounded_rect position

All other shape did it. While it usually have no side effects,
as seen in #920 screenshot from @actionless, there is instances
where this produce a invalid rectangle.

lib/gears/shape.lua

@@ -35,6 +35,7 @@ local module = {}
 -- @tparam number height The rectangle height
 -- @tparam number radius the corner radius
 function module.rounded_rect(cr, width, height, radius)
+
     radius = radius or 10
 
     if width / 2 < radius then
@@ -45,6 +46,8 @@ function module.rounded_rect(cr, width, height, radius)
         radius = height / 2
     end
 
+    cr:move_to(0, radius)
+
     cr:arc( radius      , radius       , radius,    math.pi   , 3*(math.pi/2) )
     cr:arc( width-radius, radius       , radius, 3*(math.pi/2),    math.pi*2  )
     cr:arc( width-radius, height-radius, radius,    math.pi*2 ,    math.pi/2  )