internal/shaderir: improve FilterUniformVariables

```
name      old time/op  new time/op  delta
Filter-8  31.7ns ± 1%  29.9ns ± 1%  -5.60%  (p=0.000 n=9+10)
```

Updates #2601

internal/shaderir/bench_test.go

@@ -0,0 +1,34 @@
+// Copyright 2023 The Ebitengine Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package shaderir_test
+
+import (
+	"testing"
+
+	"github.com/hajimehoshi/ebiten/v2/internal/builtinshader"
+	"github.com/hajimehoshi/ebiten/v2/internal/graphics"
+)
+
+func BenchmarkFilter(b *testing.B) {
+	src := builtinshader.Shader(builtinshader.FilterNearest, builtinshader.AddressUnsafe, false)
+	s, err := graphics.CompileShader(src)
+	if err != nil {
+		b.Fatal(err)
+	}
+	uniforms := make([]uint32, graphics.PreservedUniformUint32Count)
+	for i := 0; i < b.N; i++ {
+		s.FilterUniformVariables(uniforms)
+	}
+}

internal/shaderir/program.go

@@ -40,7 +40,7 @@ type Program struct {
 	FragmentFunc FragmentFunc
 	Unit         Unit
 
-	uniformMask []uint32
+	uniformFactors []uint32
 }
 
 type Func struct {
@@ -464,7 +464,7 @@ func (p *Program) appendReachableUniformVariablesFromBlock(indices []int, block
 // FilterUniformVariables replaces uniform variables with nil when they are not used.
 // By minimizing uniform variables, more commands can be merged in the graphicscommand package.
 func (p *Program) FilterUniformVariables(uniforms []uint32) {
-	if p.uniformMask == nil {
+	if p.uniformFactors == nil {
 		indices := p.appendReachableUniformVariablesFromBlock(nil, p.VertexFunc.Block)
 		indices = p.appendReachableUniformVariablesFromBlock(indices, p.FragmentFunc.Block)
 		reachableUniforms := make([]bool, len(p.Uniforms))
@@ -475,14 +475,14 @@ func (p *Program) FilterUniformVariables(uniforms []uint32) {
 			fs := make([]uint32, typ.Uint32Count())
 			if reachableUniforms[i] {
 				for j := range fs {
-					fs[j] = (1 << 32) - 1
+					fs[j] = 1
 				}
 			}
-			p.uniformMask = append(p.uniformMask, fs...)
+			p.uniformFactors = append(p.uniformFactors, fs...)
 		}
 	}
 
-	for i, factor := range p.uniformMask {
-		uniforms[i] &= factor
+	for i, factor := range p.uniformFactors {
+		uniforms[i] *= factor
 	}
 }