internal/f32color: add colorspace-correct function for alpha scaling

Package material's ad-hoc mulAlpha didn't take the sRGB color-space
into account, which meant that alpha-scaled colors were subtly wrong.
Introduce f32color.MulAlpha and convert all uses to it.

Thanks to René Post for finding and debugging the issue.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

widget/material/checkable.go

@@ -6,6 +6,7 @@ import (
 	"image"
 	"image/color"
 
+	"gioui.org/internal/f32color"
 	"gioui.org/io/pointer"
 	"gioui.org/layout"
 	"gioui.org/op/paint"
@@ -42,7 +43,7 @@ func (c *checkable) layout(gtx layout.Context, checked bool) layout.Dimensions {
 					size := gtx.Px(c.Size)
 					icon.Color = c.IconColor
 					if gtx.Queue == nil {
-						icon.Color = mulAlpha(icon.Color, 150)
+						icon.Color = f32color.MulAlpha(icon.Color, 150)
 					}
 					icon.Layout(gtx, unit.Px(float32(size)))
 					return layout.Dimensions{