internal/shader: some assignment operators didn't work correctly

[hajimehoshi]

Ebitengine Version

366f489

Operating System

• Windows
• macOS
• Linux
• FreeBSD
• OpenBSD
• Android
• iOS
• Nintendo Switch
• PlayStation 5
• Xbox
• Web Browsers

Go Version (go version)

go version go1.23.1 darwin/arm64

What steps will reproduce the problem?

Run this test in the ebiten package:

func TestShaderCompile(t *testing.T) {
	if _, err := ebiten.NewShader([]byte(`//kage:unit pixels

package main

func Fragment(dstPos vec4, srcPos vec2, color vec4) vec4 {
	x := 0
	x ^= x
	return vec4(0)
}
`)); err != nil {
		t.Fatal(err)
	}
}

What is the expected result?

The test passes

What happens instead?

The test fails:

--- FAIL: TestShaderCompile (0.00s)
    shader_test.go:2833: graphics: vertex shader entry point '__vertex' is missing
FAIL
FAIL    github.com/hajimehoshi/ebiten/v2        0.550s
FAIL

Anything else you feel useful to add?

No response