all: use color.NRGBA in public API

color.RGBA has two problems with regards to using it.

First the color values need to be premultiplied, whereas most APIs
have non-premultiplied values. This is mainly to preserve color components
with low alpha values.

Second there are two ways to premultiply with sRGB. One is to premultiply
after sRGB conversion, the other is before. This makes using the API more
confusing.

Using color.NRGBA in sRGB makes it align with CSS.e

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

widget/material/editor.go

@@ -18,11 +18,11 @@ type EditorStyle struct {
 	Font     text.Font
 	TextSize unit.Value
 	// Color is the text color.
-	Color color.RGBA
+	Color color.NRGBA
 	// Hint contains the text displayed when the editor is empty.
 	Hint string
 	// HintColor is the color of hint text.
-	HintColor color.RGBA
+	HintColor color.NRGBA
 	Editor    *widget.Editor
 
 	shaper text.Shaper