internal/cmd/convertshaders: move shader converter to separate package

To use the converter from other packages, make the converter a
runnable command.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

gpu/gen.go

@@ -2,4 +2,4 @@
 
 package gpu
 
-//go:generate go run build.go
+//go:generate go run ../internal/cmd/convertshaders -package gpu

internal/cmd/convertshaders/main.go

@@ -1,12 +1,11 @@
 // SPDX-License-Identifier: Unlicense OR MIT
 
-// +build ignore
-
 package main
 
 import (
 	"bytes"
 	"encoding/json"
+	"flag"
 	"fmt"
 	"go/format"
 	"io/ioutil"
@@ -22,6 +21,8 @@ import (
 // multiple GPU backends (OpenGL ES, Direct3D 11...)
 // from a single source.
 
+var packageName = flag.String("package", "", "specify Go package name")
+
 type shaderArgs struct {
 	FetchColorExpr string
 	Header         string
@@ -60,14 +61,15 @@ const (
 )
 
 func main() {
+	flag.Parse()
 	if err := generate(); err != nil {
-		fmt.Fprintf(os.Stderr, "gpu generate: %v\n", err)
+		fmt.Fprintf(os.Stderr, "generate: %v\n", err)
 		os.Exit(1)
 	}
 }
 
 func generate() error {
-	tmp, err := ioutil.TempDir("", "gpu-generate")
+	tmp, err := ioutil.TempDir("", "shader-convert")
 	if err != nil {
 		return err
 	}
@@ -84,7 +86,7 @@ func generate() error {
 	}
 	var out bytes.Buffer
 	out.WriteString("// Code generated by build.go. DO NOT EDIT.\n\n")
-	out.WriteString("package gpu\n\n")
+	fmt.Fprintf(&out, "package %s\n\n", *packageName)
 	fmt.Fprintf(&out, "import %q\n\n", "gioui.org/gpu/backend")
 
 	out.WriteString("var (\n")