precomputed shade algo, viewport basic render

ui/mainwindow/layer.go

@@ -36,10 +36,14 @@ func (layer *Layer) WithColor(colorName string) *Layer {
 func (layer *Layer) PutWithRawBackground(x,y int, symbol interface{}, bgColor uint32) {
 	layer.before()
 	prevColor := uint32(blt.State(blt.TK_COLOR))
+	prevCompMode := blt.State(blt.TK_COMPOSITION)
 	blt.Color(bgColor)
+	blt.Composition(1)
 	layer.Put(x,y,"█")
 	blt.Color(prevColor)
 	layer.Put(x,y, symbol)
+	blt.Composition(prevCompMode)
+
 }
 
 func (layer *Layer) PutWithBackground(x,y int, symbol interface{}, bgColorName string) {