iOS: Add support for offscreen GL surfaces without using hidden windows

There's no need to create a hidden window to get a surface on iOS, as
the platform supports FBOs. Note that defaultFramebufferObject() returns
0 in the case of offscreen surfaces, which is technically not a valid
FBO on iOS due to the indirect rendering, but binding and rendering
to the zero-FBO seems to be no-ops, so clients may safely call eg
glBindFramebuffer(GL_FRAMEBUFFER, ctx->defaultFramebufferObject())
to restore the default FBO after drawing to its own FBO.

Change-Id: I2e67f5d69c0698562052f5ac1df0bbfaa3337148
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>

src/plugins/platforms/ios/qioscontext.mm

@@ -113,6 +113,11 @@ bool QIOSContext::makeCurrent(QPlatformSurface *surface)
     Q_ASSERT(surface && surface->surface()->surfaceType() == QSurface::OpenGLSurface);
 
     [EAGLContext setCurrentContext:m_eaglContext];
+
+    // For offscreen surfaces we don't prepare a default FBO
+    if (surface->surface()->surfaceClass() == QSurface::Offscreen)
+        return true;
+
     FramebufferObject &framebufferObject = backingFramebufferObjectFor(surface);
 
     // We bind the default FBO even if it's incomplete, so that clients who
@@ -131,6 +136,10 @@ void QIOSContext::doneCurrent()
 void QIOSContext::swapBuffers(QPlatformSurface *surface)
 {
     Q_ASSERT(surface && surface->surface()->surfaceType() == QSurface::OpenGLSurface);
+
+    if (surface->surface()->surfaceClass() == QSurface::Offscreen)
+        return; // Nothing to do
+
     Q_ASSERT(surface->surface()->surfaceClass() == QSurface::Window);
     QIOSWindow *window = static_cast<QIOSWindow *>(surface);
     Q_ASSERT(m_framebufferObjects.contains(window));
@@ -213,6 +222,13 @@ QIOSContext::FramebufferObject &QIOSContext::backingFramebufferObjectFor(QPlatfo
 
 GLuint QIOSContext::defaultFramebufferObject(QPlatformSurface *surface) const
 {
+    if (surface->surface()->surfaceClass() == QSurface::Offscreen) {
+        // Binding and rendering to the zero-FBO on iOS seems to be
+        // no-ops, so we can safely return 0 here, even if it's not
+        // really a valid FBO on iOS.
+        return 0;
+    }
+
     return backingFramebufferObjectFor(surface).handle;
 }
 

src/plugins/platforms/ios/qiosintegration.h

@@ -64,6 +64,7 @@ public:
     QPlatformBackingStore *createPlatformBackingStore(QWindow *window) const;
 
     QPlatformOpenGLContext *createPlatformOpenGLContext(QOpenGLContext *context) const;
+    QPlatformOffscreenSurface *createPlatformOffscreenSurface(QOffscreenSurface *surface) const Q_DECL_OVERRIDE;
 
     QPlatformFontDatabase *fontDatabase() const;
     QPlatformClipboard *clipboard() const;

src/plugins/platforms/ios/qiosintegration.mm

@@ -51,6 +51,8 @@
 #include "qiostheme.h"
 #include "qiosservices.h"
 
+#include <qpa/qplatformoffscreensurface.h>
+
 #include <QtPlatformSupport/private/qcoretextfontdatabase_p.h>
 #include <QtPlatformSupport/private/qmacmime_p.h>
 #include <QDir>
@@ -144,6 +146,11 @@ QPlatformOpenGLContext *QIOSIntegration::createPlatformOpenGLContext(QOpenGLCont
     return new QIOSContext(context);
 }
 
+QPlatformOffscreenSurface *QIOSIntegration::createPlatformOffscreenSurface(QOffscreenSurface *surface) const
+{
+    return new QPlatformOffscreenSurface(surface);
+}
+
 QAbstractEventDispatcher *QIOSIntegration::createEventDispatcher() const
 {
     if (isQtApplication())