iOS: Set context and bind FBO before allocating render-buffer storage

defaultFramebufferObject() may be called from anywhere, at any point,
not just makeCurrent(). One example is the glyph-cache, which uses it
to re-bind the default FBO after generating the texture cache.

If the default FBO had already been created, but the render-buffer was
out of sync with the window size, we would end up in the resize code
without the correct context current, and without the render-buffer's
owning FBO bound. This caused "Failed to make complete framebuffer
object 8cd7" warnings at runtime.

We now make the context current and bind the FBO, even though it might
already be bound and the context current from makeCurrent(), or when
initially creating the FBO. For the future we should move the whole
resize logic out of defaultFramebufferObject() and call it from
makeCurrent(), or possibly [EAGLView layoutSubviews]. That's a
higher impact change though, which we reserve for the 'dev' branch.

Change-Id: I50ea949c12a02ad1af6ec9fdc3215d5da85b324f
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>

src/plugins/platforms/ios/qioscontext.mm

@@ -156,6 +156,9 @@ GLuint QIOSContext::defaultFramebufferObject(QPlatformSurface *surface) const
     if (framebufferObject.renderbufferWidth != platformWindow->effectiveWidth() ||
         framebufferObject.renderbufferHeight != platformWindow->effectiveHeight()) {
 
+        [EAGLContext setCurrentContext:m_eaglContext];
+        glBindFramebuffer(GL_FRAMEBUFFER, framebufferObject.handle);
+
         glBindRenderbuffer(GL_RENDERBUFFER, framebufferObject.colorRenderbuffer);
         UIView *view = reinterpret_cast<UIView *>(platformWindow->winId());
         CAEAGLLayer *layer = static_cast<CAEAGLLayer *>(view.layer);