tpl: Add float template function

Add a template function that allows conversion to float. This is
useful, for example, when passing aspect ratios into templates,
which tend to not be integers.

Fixes #3307

tpl/cast/cast_test.go

@@ -81,3 +81,40 @@ func TestToString(t *testing.T) {
 		assert.Equal(t, test.expect, result, errMsg)
 	}
 }
+
+func TestToFloat(t *testing.T) {
+	t.Parallel()
+
+	ns := New()
+
+	for i, test := range []struct {
+		v      interface{}
+		expect interface{}
+	}{
+		{"1", 1.0},
+		{template.HTML("2"), 2.0},
+		{template.CSS("3"), 3.0},
+		{template.HTMLAttr("4"), 4.0},
+		{template.JS("-5.67"), -5.67},
+		{template.JSStr("6"), 6.0},
+		{"1.23", 1.23},
+		{"-1.23", -1.23},
+		{"0", 0.0},
+		{float64(2.12), 2.12},
+		{int64(123), 123.0},
+		{2, 2.0},
+		{t, false},
+	} {
+		errMsg := fmt.Sprintf("[%d] %v", i, test.v)
+
+		result, err := ns.ToFloat(test.v)
+
+		if b, ok := test.expect.(bool); ok && !b {
+			require.Error(t, err, errMsg)
+			continue
+		}
+
+		require.NoError(t, err, errMsg)
+		assert.Equal(t, test.expect, result, errMsg)
+	}
+}