MaskView: Masks with multiple draw calls break nested masks

[benjaminbojko]

If a view that produces multiple gl draw calls is used as a MaskView mask, it will increment the stencil buffer multiple times and the stencil test will break.

This can be triggered by a nested mask, where the child MaskView has an ImageView (because it calls both BaseView::draw() and gl::draw(mTexture, ...)) or a BaseView with multiple children as a mask.