Silent failure on dst image type different than *image.NRGBA

[Krzysztofz01]

Hello (it's me again)

I have found one bug that can be difficult to debug, due to its runtime nature and silent failure. Currently, there is no error handling for a case where the dst image is not a *image.NRGBA. A succesfull operation when passing *image.NRGBA as dst and a case when (for example) *image.RGBA is passed as dst can not be distinguished by the return value.

I have added a type validation for the dst image and a comment guiding the developer to provide an image.Image which is a *image.NRGBA internally.

PoC:

func TestPoc(t *testing.T) {
	nrgbaImg := image.NewNRGBA(image.Rect(0, 0, 10, 10))
	rgbaImg := image.NewRGBA(image.Rect(0, 0, 10, 10))

	err := Process(rgbaImg, nrgbaImg, 10)

	if err == nil {
		t.Error("Rgba was used as the destination but no error was returned.")
	}
}

Best regards,
Krzysztof Zoń