Fix grid widths when using weighted widths

If the column weights do not collectively divide the extent of the grid
layout then some width was not used and so would not be redrawn,
resulting in previous drawings showing through.

This fixes this by checking if there is any remainingExact width and if
there is it is assigned to the weighted columns by their proportion from
left to right.

lib/ui/grid.go

@@ -134,14 +134,25 @@ func (grid *Grid) reflow(ctx *Context) {
 			}
 		}
 		offset := 0
+		remainingExact := 0
+		if weight > 0 {
+			remainingExact = (extent - exact) % weight
+		}
 		for _, spec := range *specs {
 			layout := gridLayout{Offset: offset}
 			if spec.Strategy == SIZE_EXACT {
 				layout.Size = spec.Size
 			} else if spec.Strategy == SIZE_WEIGHT {
-				size := float64(spec.Size) / float64(weight)
-				size *= float64(extent - exact)
-				layout.Size = int(math.Floor(size))
+				proportion := float64(spec.Size) / float64(weight)
+				size := proportion * float64(extent-exact)
+				if remainingExact > 0 {
+					extraExact := int(math.Ceil(proportion * float64(remainingExact)))
+					layout.Size = int(math.Floor(size)) + extraExact
+					remainingExact -= extraExact
+
+				} else {
+					layout.Size = int(math.Floor(size))
+				}
 			}
 			offset += layout.Size
 			*layouts = append(*layouts, layout)