shader: Bug fix: a blank identifier lhs and += cannot work

internal/shader/stmt.go

@@ -81,7 +81,7 @@ func (cs *compileState) parseStmt(block *block, fname string, stmt ast.Stmt, inP
 			}
 			stmts = append(stmts, ss...)
 
-			lhs, ts, ss, ok := cs.parseExpr(block, stmt.Lhs[0], false)
+			lhs, ts, ss, ok := cs.parseExpr(block, stmt.Lhs[0], true)
 			if !ok {
 				return nil, false
 			}

shader_test.go

@@ -642,6 +642,16 @@ func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
 	_++
 	return vec4(0)
 }
+`)); err == nil {
+		t.Errorf("error must be non-nil but was nil")
+	}
+
+	if _, err := NewShader([]byte(`package main
+
+func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
+	_ += 1
+	return vec4(0)
+}
 `)); err == nil {
 		t.Errorf("error must be non-nil but was nil")
 	}