graphicsdriver/opengl: Bug fix: Binding the same texture multiple times did not work

Updates #1193

internal/graphicsdriver/opengl/program.go

@@ -284,17 +284,43 @@ func (g *Graphics) useProgram(program program, uniforms []uniformVariable, textu
 		}
 	}
 
+	type activatedTexture struct {
+		textureNative textureNative
+		index         int
+	}
+
+	// textureNative cannot be a map key unfortunately.
+	textureToActivatedTexture := []activatedTexture{}
+	var idx int
+loop:
 	for i, t := range textures {
 		if !t.valid {
 			continue
 		}
-		g.context.uniformInt(program, fmt.Sprintf("T%d", i), i)
-		if g.state.lastActiveTexture != i {
-			g.context.activeTexture(i)
-			g.state.lastActiveTexture = i
+
+		// If the texture is already bound, set the texture variable to point to the texture.
+		// Rebinding the same texture seems problematic (#1193).
+		for _, at := range textureToActivatedTexture {
+			if t.native.equal(at.textureNative) {
+				g.context.uniformInt(program, fmt.Sprintf("T%d", i), at.index)
+				continue loop
 			}
+		}
+
+		textureToActivatedTexture = append(textureToActivatedTexture, activatedTexture{
+			textureNative: t.native,
+			index:         idx,
+		})
+		g.context.uniformInt(program, fmt.Sprintf("T%d", i), idx)
+		if g.state.lastActiveTexture != idx {
+			g.context.activeTexture(idx)
+			g.state.lastActiveTexture = idx
+		}
+
 		// Apparently, a texture must be bound every time. The cache is not used here.
 		g.context.bindTexture(t.native)
+
+		idx++
 	}
 
 	return nil

internal/restorable/shader_test.go

@@ -130,13 +130,13 @@ func TestShaderMultipleSourcesOnOneTexture(t *testing.T) {
 		t.Skip("shader is not available on this environment")
 	}
 
-	var srcs [graphics.ShaderImageNum]*Image
-	srcs[0] = NewImage(3, 1, false)
-	srcs[0].ReplacePixels([]byte{
+	src := NewImage(3, 1, false)
+	src.ReplacePixels([]byte{
 		0x40, 0, 0, 0xff,
 		0, 0x80, 0, 0xff,
 		0, 0, 0xc0, 0xff,
 	}, 0, 0, 3, 1)
+	srcs := [graphics.ShaderImageNum]*Image{src, src, src}
 
 	dst := NewImage(1, 1, false)