iOS: if `MTKView` can't be drawn to, keep the resize/redraw "flag" dirty (#1100)

There was a bug where apps on iPad would not always be properly redrawn
when being resized (in windowed mode). This fixes that (at least in my testing)
by NOT clearing the dirty state when the MTKView has no render pass descriptor

Thus, the next time it's valid, we repaint it as expected.