internal/graphicsdriver/opengl: initialize gl at init()

This changes context_gl.go a little closer to context_gles.go.

Updates #2451

internal/graphicsdriver/opengl/context_gl.go

@@ -83,18 +83,9 @@ func getProgramID(p program) programID {
 }
 
 type contextImpl struct {
-	init bool
 }
 
 func (c *context) reset() error {
-	if !c.init {
-		// Note that this initialization must be done after Loop is called.
-		if err := gl.Init(); err != nil {
-			return fmt.Errorf("opengl: initializing error %v", err)
-		}
-		c.init = true
-	}
-
 	c.locationCache = newLocationCache()
 	c.lastTexture = invalidTexture
 	c.lastFramebuffer = invalidFramebuffer

internal/graphicsdriver/opengl/graphics.go

@@ -30,7 +30,9 @@ func NewGraphics() (graphicsdriver.Graphics, error) {
 		return nil, fmt.Errorf("opengl: OpenGL is not supported on Xbox")
 	}
 	g := &Graphics{}
-	g.init()
+	if err := g.init(); err != nil {
+		return nil, err
+	}
 	return g, nil
 }
 

internal/graphicsdriver/opengl/graphics_gl.go

@@ -16,6 +16,10 @@
 
 package opengl
 
-func (g *Graphics) init() {
-	// Do nothing.
+import (
+	"github.com/hajimehoshi/ebiten/v2/internal/graphicsdriver/opengl/gl"
+)
+
+func (g *Graphics) init() error {
+	return gl.Init()
 }

internal/graphicsdriver/opengl/graphics_gles.go

@@ -20,6 +20,7 @@ import (
 	"github.com/hajimehoshi/ebiten/v2/internal/graphicsdriver/opengl/gles"
 )
 
-func (g *Graphics) init() {
+func (g *Graphics) init() error {
 	g.context.ctx = gles.DefaultContext{}
+	return nil
 }

internal/graphicsdriver/opengl/graphics_js.go

@@ -18,8 +18,9 @@ import (
 	"syscall/js"
 )
 
-func (g *Graphics) init() {
+func (g *Graphics) init() error {
 	// Do nothing.
+	return nil
 }
 
 func (g *Graphics) SetCanvas(canvas js.Value) {