Add a test when a single image is used with DrawMRT

internal/graphicsdriver/directx/graphics12_windows.go

@@ -1225,7 +1225,7 @@ func (g *graphics12) DrawTriangles(dstIDs [graphics.ShaderDstImageCount]graphics
 	// If the number of targets is more than one, or if the only target is the first one, then
 	// it is safe to assume that MRT is used.
 	// Also, it only matters in order to specify empty targets/viewports when not all slots are
-	// being filled.
+	// being filled, even though it's not a MRT scenario.
 	usesMRT := targetCount > 1 || firstTarget > 0
 	if usesMRT {
 		targetCount = graphics.ShaderDstImageCount

shader_test.go

@@ -2677,6 +2677,7 @@ func Fragment(dstPos vec4, srcPos vec2, color vec4) (vec4, vec4, vec4, vec4, vec
 		}
 	})
 
+	// Clear images
 	for _, img := range imgs {
 		img.Clear()
 	}
@@ -2707,4 +2708,30 @@ func Fragment(dstPos vec4, srcPos vec2, color vec4) (vec4, vec4, vec4, vec4, vec
 			}
 		}
 	})
+
+	// Clear images
+	for _, img := range imgs {
+		img.Clear()
+	}
+	t.Run("1 location (first slot)", func(t *testing.T) {
+		wantColors := [8]color.RGBA{
+			{R: 0xff, G: 0, B: 0, A: 0xff},
+			{}, {}, {}, {}, {}, {}, {},
+		}
+		dsts := [8]*ebiten.Image{
+			imgs[0], nil, nil, nil, nil, nil, nil, nil,
+		}
+		ebiten.DrawTrianglesShaderMRT(dsts, vertices, indices, s, nil)
+		for k, dst := range imgs {
+			for j := 0; j < h; j++ {
+				for i := 0; i < w; i++ {
+					got := dst.At(i, j).(color.RGBA)
+					want := wantColors[k]
+					if !sameColors(got, want, 1) {
+						t.Errorf("dst.At(%d, %d): got: %v, want: %v", i, j, got, want)
+					}
+				}
+			}
+		}
+	})
 }