all: embed texture file

This makes the result binary more portable. It can be copied to another
machine where the source code isn't present and still run okay. Relying
on the go/build package and GOPATH to find assets was the best we could
do before the module mode.

Fixes #85.

gl21-cube/cube.go

@@ -2,16 +2,15 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Renders a textured spinning cube using GLFW 3 and OpenGL 2.1.
-package main // import "github.com/go-gl/example/gl21-cube"
+// gl21-cube renders a textured spinning cube using GLFW 3 and OpenGL 2.1.
+package main
 
 import (
-	"go/build"
+	"embed"
 	"image"
 	"image/draw"
 	_ "image/png"
 	"log"
-	"os"
 	"runtime"
 
 	"github.com/go-gl/gl/v2.1/gl"
@@ -62,7 +61,7 @@ func main() {
 }
 
 func newTexture(file string) uint32 {
-	imgFile, err := os.Open(file)
+	imgFile, err := assetFS.Open(file)
 	if err != nil {
 		log.Fatalf("texture %q not found on disk: %v\n", file, err)
 	}
@@ -204,25 +203,5 @@ func drawScene() {
 	gl.End()
 }
 
-// Set the working directory to the root of Go package, so that its assets can be accessed.
-func init() {
-	dir, err := importPathToDir("github.com/go-gl/example/gl21-cube")
-	if err != nil {
-		log.Fatalln("Unable to find Go package in your GOPATH, it's needed to load assets:", err)
-	}
-	err = os.Chdir(dir)
-	if err != nil {
-		log.Panicln("os.Chdir:", err)
-	}
-}
-
-// importPathToDir resolves the absolute path from importPath.
-// There doesn't need to be a valid Go package inside that import path,
-// but the directory must exist.
-func importPathToDir(importPath string) (string, error) {
-	p, err := build.Import(importPath, "", build.FindOnly)
-	if err != nil {
-		return "", err
-	}
-	return p.Dir, nil
-}
+//go:embed square.png
+var assetFS embed.FS

gl41core-cube/cube.go

@@ -2,17 +2,16 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Renders a textured spinning cube using GLFW 3 and OpenGL 4.1 core forward-compatible profile.
-package main // import "github.com/go-gl/example/gl41core-cube"
+// gl41core-cube renders a textured spinning cube using GLFW 3 and OpenGL 4.1 core forward-compatible profile.
+package main
 
 import (
+	"embed"
 	"fmt"
-	"go/build"
 	"image"
 	"image/draw"
 	_ "image/png"
 	"log"
-	"os"
 	"runtime"
 	"strings"
 
@@ -198,7 +197,7 @@ func compileShader(source string, shaderType uint32) (uint32, error) {
 }
 
 func newTexture(file string) (uint32, error) {
-	imgFile, err := os.Open(file)
+	imgFile, err := assetFS.Open(file)
 	if err != nil {
 		return 0, fmt.Errorf("texture %q not found on disk: %v", file, err)
 	}
@@ -318,25 +317,5 @@ var cubeVertices = []float32{
 	1.0, 1.0, 1.0, 0.0, 1.0,
 }
 
-// Set the working directory to the root of Go package, so that its assets can be accessed.
-func init() {
-	dir, err := importPathToDir("github.com/go-gl/example/gl41core-cube")
-	if err != nil {
-		log.Fatalln("Unable to find Go package in your GOPATH, it's needed to load assets:", err)
-	}
-	err = os.Chdir(dir)
-	if err != nil {
-		log.Panicln("os.Chdir:", err)
-	}
-}
-
-// importPathToDir resolves the absolute path from importPath.
-// There doesn't need to be a valid Go package inside that import path,
-// but the directory must exist.
-func importPathToDir(importPath string) (string, error) {
-	p, err := build.Import(importPath, "", build.FindOnly)
-	if err != nil {
-		return "", err
-	}
-	return p.Dir, nil
-}
+//go:embed square.png
+var assetFS embed.FS