shader: Forbid init functions

Fixes #1331

internal/shader/shader.go

@@ -565,6 +565,10 @@ func (cs *compileState) parseFunc(block *block, d *ast.FuncDecl) (function, bool
 		cs.addError(d.Pos(), "function must have a name")
 		return function{}, false
 	}
+	if d.Name.Name == "init" {
+		cs.addError(d.Pos(), "init function is not implemented")
+		return function{}, false
+	}
 	if d.Body == nil {
 		cs.addError(d.Pos(), "function must have a body")
 		return function{}, false

shader_test.go

@@ -358,3 +358,17 @@ func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
 		t.Error(err)
 	}
 }
+
+func TestShaderInit(t *testing.T) {
+	if _, err := NewShader([]byte(`package main
+
+func init() {
+}
+
+func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
+	return vec4(0)
+}
+`)); err == nil {
+		t.Errorf("error must be non-nil but was nil")
+	}
+}