fixes

pixelgl/shader.go

@@ -17,6 +17,9 @@ type Shader struct {
 //
 // Note that vertexShader and fragmentShader parameters must contain the source code, they're not filenames.
 func NewShader(parent BeginEnder, vertexShader, fragmentShader string) (*Shader, error) {
+	parent.Begin()
+	defer parent.End()
+
 	shader := &Shader{
 		parent: parent,
 	}

pixelgl/texture.go

@@ -14,6 +14,9 @@ type Texture struct {
 // NewTexture creates a new texture with the specified width and height.
 // The pixels must be a sequence of RGBA values.
 func NewTexture(parent BeginEnder, width, height int, pixels []uint8) (*Texture, error) {
+	parent.Begin()
+	defer parent.End()
+
 	texture := &Texture{parent: parent}
 	err := DoGLErr(func() {
 		gl.GenTextures(1, &texture.tex)

pixelgl/vertex.go

@@ -100,6 +100,9 @@ type VertexArray struct {
 
 // NewVertexArray creates a new vertex array and wraps another BeginEnder around it.
 func NewVertexArray(parent BeginEnder, format VertexFormat, mode VertexDrawMode, usage VertexUsage, data []float64) (*VertexArray, error) {
+	parent.Begin()
+	defer parent.End()
+
 	va := &VertexArray{
 		parent: parent,
 		format: format,

window.go

@@ -2,6 +2,7 @@ package pixel
 
 import (
 	"github.com/faiface/pixel/pixelgl"
+	"github.com/go-gl/gl/v3.3-core/gl"
 	"github.com/go-gl/glfw/v3.2/glfw"
 	"github.com/pkg/errors"
 )
@@ -59,26 +60,38 @@ func NewWindow(config WindowConfig) (*Window, error) {
 	return w, nil
 }
 
-func (w *Window) Update() {
+func (w *Window) Clear(r, g, b, a float64) {
+	w.Begin()
+	pixelgl.Do(func() {
+		gl.ClearColor(float32(r), float32(g), float32(b), float32(a))
+		gl.Clear(gl.COLOR_BUFFER_BIT)
+	})
+	w.End()
+}
+
+func (w *Window) Update() {
+	w.Begin()
 	pixelgl.Do(func() {
-		w.Begin()
 		if w.config.VSync {
 			glfw.SwapInterval(1)
 		}
 		w.window.SwapBuffers()
 		glfw.PollEvents()
-		w.End()
 	})
+	w.End()
 }
 
 var currentWindow *Window = nil
 
 func (w *Window) Begin() {
-	if currentWindow != w {
-		w.window.MakeContextCurrent()
-		pixelgl.Init()
-		currentWindow = w
-	}
+	pixelgl.Do(func() {
+		if currentWindow != w {
+			w.window.MakeContextCurrent()
+			pixelgl.Init()
+			currentWindow = w
+		}
+	})
+
 }
 
 func (w *Window) End() {