fix ios11 black screen when using postprocess (#1333)

ios11 requires to flush transactions before creating a new
framebuffer for full screen postproceses.

src/glcontext_eagl.mm

@@ -189,6 +189,7 @@ namespace bgfx { namespace gl
 
 		m_context = (void*)context;
 		[EAGLContext setCurrentContext:context];
+		[CATransaction flush];
 
 		GL_CHECK(glGenFramebuffers(1, &m_fbo) );
 		GL_CHECK(glBindFramebuffer(GL_FRAMEBUFFER, m_fbo) );