app/headless: prepare OpenGL context on macOS

The macOS backend uses a desktop OpenGL context, not a OpenGL ES
context. The main difference is that sRGB have to be enabled and
a vertex buffer must be bound.

Do that and fix Window.Screenshot for scenes more complex than a
glClear.

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

app/headless/headless_darwin.go

@@ -13,8 +13,9 @@ import "gioui.org/app/internal/gl"
 import "C"
 
 type nsContext struct {
-	c   *gl.Functions
-	ctx C.CFTypeRef
+	c        *gl.Functions
+	ctx      C.CFTypeRef
+	prepared bool
 }
 
 func newContext() (context, error) {
@@ -24,6 +25,10 @@ func newContext() (context, error) {
 
 func (c *nsContext) MakeCurrent() error {
 	C.gio_headless_makeCurrentContext(c.ctx)
+	if !c.prepared {
+		C.gio_headless_prepareContext(c.ctx)
+		c.prepared = true
+	}
 	return nil
 }
 

app/headless/headless_darwin.h

@@ -4,3 +4,4 @@ __attribute__ ((visibility ("hidden"))) CFTypeRef gio_headless_newContext(void);
 __attribute__ ((visibility ("hidden"))) void gio_headless_releaseContext(CFTypeRef ctxRef);
 __attribute__ ((visibility ("hidden"))) void gio_headless_clearCurrentContext(CFTypeRef ctxRef);
 __attribute__ ((visibility ("hidden"))) void gio_headless_makeCurrentContext(CFTypeRef ctxRef);
+__attribute__ ((visibility ("hidden"))) void gio_headless_prepareContext(CFTypeRef ctxRef);

app/headless/headless_ios.m

@@ -27,3 +27,6 @@ void gio_headless_makeCurrentContext(CFTypeRef ctxRef) {
 	EAGLContext *ctx = (__bridge EAGLContext *)ctxRef;
 	[EAGLContext setCurrentContext:ctx];
 }
+
+void gio_headless_prepareContext(CFTypeRef ctxRef) {
+}

app/headless/headless_macos.m

@@ -5,6 +5,7 @@
 @import AppKit;
 @import OpenGL;
 @import OpenGL.GL;
+@import OpenGL.GL3;
 
 #include <CoreFoundation/CoreFoundation.h>
 #include "headless_darwin.h"
@@ -40,6 +41,13 @@ void gio_headless_clearCurrentContext(CFTypeRef ctxRef) {
 void gio_headless_makeCurrentContext(CFTypeRef ctxRef) {
 	NSOpenGLContext *ctx = (__bridge NSOpenGLContext *)ctxRef;
 	[ctx makeCurrentContext];
-	glEnable(GL_FRAMEBUFFER_SRGB);
 	CGLLockContext([ctx CGLContextObj]);
 }
+
+void gio_headless_prepareContext(CFTypeRef ctxRef) {
+	// Bind a default VBA to emulate OpenGL ES 2.
+	GLuint defVBA;
+	glGenVertexArrays(1, &defVBA);
+	glBindVertexArray(defVBA);
+	glEnable(GL_FRAMEBUFFER_SRGB);
+}

app/headless/headless_test.go

@@ -22,9 +22,10 @@ func TestHeadless(t *testing.T) {
 	col := color.RGBA{A: 0xff, R: 0xcc, G: 0xcc}
 	var ops op.Ops
 	paint.ColorOp{Color: col}.Add(&ops)
+	// Paint only part of the screen to avoid the glClear optimization.
 	paint.PaintOp{Rect: f32.Rectangle{Max: f32.Point{
-		X: float32(sz.X),
-		Y: float32(sz.Y),
+		X: float32(sz.X) - 100,
+		Y: float32(sz.Y) - 100,
 	}}}.Add(&ops)
 	w.Frame(&ops)