x/exp/shiny/driver/gldriver: allow ES2 on Linux and ES3 on Windows

The previous check for GL_ES_2_0 was compile-time, which makes no sense.
The lack of support for OpenGL ES 2.0 contexts on Linux also makes no
sense as gldriver supports OpenGL ES 2.0 on other platforms.

shiny/driver/gldriver/win32.go

@@ -329,7 +329,7 @@ func createEGLSurface(hwnd syscall.Handle, w *windowImpl) error {
 	}
 
 	contextAttribs := [...]eglInt{
-		_EGL_CONTEXT_CLIENT_VERSION, 2,
+		_EGL_CONTEXT_CLIENT_VERSION, 3,
 		_EGL_NONE,
 	}
 	context, _, _ := eglCreateContext.Call(
@@ -338,6 +338,19 @@ func createEGLSurface(hwnd syscall.Handle, w *windowImpl) error {
 		_EGL_NO_CONTEXT,
 		uintptr(unsafe.Pointer(&contextAttribs[0])),
 	)
+	if context == _EGL_NO_CONTEXT {
+		// Try again with OpenGL ES 2.0.
+		context2Attribs := [...]eglInt{
+			_EGL_CONTEXT_CLIENT_VERSION, 2,
+			_EGL_NONE,
+		}
+		context, _, _ = eglCreateContext.Call(
+			display,
+			uintptr(config),
+			_EGL_NO_CONTEXT,
+			uintptr(unsafe.Pointer(&context2Attribs[0])),
+		)
+	}
 	if context == _EGL_NO_CONTEXT {
 		return fmt.Errorf("eglCreateContext failed: %v", eglErr())
 	}

shiny/driver/gldriver/x11.c

@@ -126,6 +126,15 @@ startDriver() {
 		EGL_NONE
 	};
 	e_ctx = eglCreateContext(e_dpy, e_config, EGL_NO_CONTEXT, ctx_attribs);
+	if (!e_ctx) {
+		// Try again with OpenGL ES 2.0.
+		static const EGLint ctx2_attribs[] = {
+			EGL_CONTEXT_CLIENT_VERSION, 2,
+			EGL_NONE
+		};
+
+		e_ctx = eglCreateContext(e_dpy, e_config, EGL_NO_CONTEXT, ctx2_attribs);
+	}
 	if (!e_ctx) {
 		fprintf(stderr, "eglCreateContext failed: %s\n", eglGetErrorStr());
 		exit(1);

shiny/driver/gldriver/x11.go

@@ -130,9 +130,6 @@ type uiClosure struct {
 }
 
 func main(f func(screen.Screen)) error {
-	if gl.Version() == "GL_ES_2_0" {
-		return errors.New("gldriver: ES 3 required on X11")
-	}
 	C.startDriver()
 	glctx, worker = gl.NewContext()