Fill background for borderless one-line paragraph

Previously when we had a Par with Width > len(Text) and custom BgColor
on the underlying Block, it would skip the first row. This means that
it's impossible to have a colored line wider than text width.
Block should offset indexes by 1 only if border is present.

block.go

@@ -92,8 +92,12 @@ func (d *Block) Buffer() []Point {
 	for i := 0; i < d.innerWidth; i++ {
 		for j := 0; j < d.innerHeight; j++ {
 			p := Point{}
-			p.X = d.X + 1 + i
-			p.Y = d.Y + 1 + j
+			p.X = d.X + i
+			p.Y = d.Y + j
+			if d.HasBorder {
+				p.X++
+				p.Y++
+			}
 			p.Ch = ' '
 			p.Bg = d.BgColor
 			ps = append(ps, p)

p_test.go

@@ -0,0 +1,20 @@
+package termui
+
+import "testing"
+
+func TestPar_NoBorderBackground(t *testing.T) {
+	par := NewPar("a")
+	par.HasBorder = false
+	par.BgColor = ColorBlue
+	par.TextBgColor = ColorBlue
+	par.Width = 2
+	par.Height = 2
+
+	pts := par.Buffer()
+	for _, p := range pts {
+		t.Log(p)
+		if p.Bg != par.BgColor {
+			t.Errorf("expected color to be %v but got %v", par.BgColor, p.Bg)
+		}
+	}
+}