add pixel to avoid empty image

internal/epub/filters/pixel.go

@@ -0,0 +1,32 @@
+package filters
+
+import (
+	"image"
+	"image/color"
+	"image/draw"
+
+	"github.com/disintegration/gift"
+)
+
+func Pixel() gift.Filter {
+	return &pixel{}
+}
+
+type pixel struct {
+}
+
+func (p *pixel) Bounds(srcBounds image.Rectangle) (dstBounds image.Rectangle) {
+	if srcBounds.Dx() == 0 || srcBounds.Dy() == 0 {
+		dstBounds = image.Rect(0, 0, 1, 1)
+	} else {
+		dstBounds = srcBounds
+	}
+	return
+}
+
+func (p *pixel) Draw(dst draw.Image, src image.Image, options *gift.Options) {
+	if dst.Bounds().Dx() == 1 && dst.Bounds().Dy() == 1 {
+		dst.Set(0, 0, color.White)
+	}
+	draw.Draw(dst, dst.Bounds(), src, src.Bounds().Min, draw.Src)
+}