shader: Add array initialization

Updates #1235

internal/shader/testdata/array.expected.vs

@@ -1 +1,9 @@
 uniform vec2[4] U0;
+
+void F0(out vec2[2] l0);
+
+void F0(out vec2[2] l0) {
+	vec2[2] l1 = vec2[2](vec2(0), vec2(0));
+	l0 = l1;
+	return;
+}

internal/shader/testdata/array.go

@@ -1,3 +1,8 @@
 package main
 
 var Array [4]vec2
+
+func Foo() [2]vec2 {
+	var x [2]vec2
+	return x
+}

internal/shaderir/glsl.go

@@ -195,7 +195,12 @@ func (p *Program) glslVarInit(t *Type) string {
 	case None:
 		return "?(none)"
 	case Array:
-		panic("not implemented")
+		init := p.glslVarInit(&t.Sub[0])
+		es := make([]string, 0, t.Length)
+		for i := 0; i < t.Length; i++ {
+			es = append(es, init)
+		}
+		return fmt.Sprintf("%s[%d](%s)", t.Sub[0].Glsl(), t.Length, strings.Join(es, ", "))
 	case Struct:
 		panic("not implemented")
 	case Bool: