unit: add Metric.DpToSp and Metric.SpToDp

It's sometimes necessary to specify padding or spacing based on
the text size.

Signed-off-by: Egon Elbre <egonelbre@gmail.com>

unit/unit_test.go

@@ -0,0 +1,32 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package unit_test
+
+import (
+	"testing"
+
+	"gioui.org/unit"
+)
+
+func TestMetric_DpToSp(t *testing.T) {
+	m := unit.Metric{
+		PxPerDp: 2,
+		PxPerSp: 3,
+	}
+
+	{
+		exp := m.Dp(5)
+		got := m.Sp(m.DpToSp(5))
+		if got != exp {
+			t.Errorf("DpToSp conversion mismatch %v != %v", exp, got)
+		}
+	}
+
+	{
+		exp := m.Sp(5)
+		got := m.Dp(m.SpToDp(5))
+		if got != exp {
+			t.Errorf("SpToDp conversion mismatch %v != %v", exp, got)
+		}
+	}
+}