Fbo: with_stencilbuffer parameter.

Adds a stencil buffer to fbo if a new option is set to True.

Example of code that demonstrate stencil instructions on fbo:
```python
from kivy.app import App
from kivy.uix.widget import Widget
from kivy.graphics import Color, Rectangle, Fbo, StencilPush, StencilPop, StencilUse, StencilUnUse
 
 
class TestWidget(Widget):
 
    def __init__(self, **kwargs):
        super(TestWidget, self).__init__(**kwargs)
 
        # Create Fbo with a stencil buffer
        with self.canvas:
            self.my_fbo = Fbo(size=self.size, with_stencilbuffer=True)
 
        # Add Fbo texture to canvas
        with self.canvas:
            Color()
            Rectangle(pos=self.pos, size=self.size, texture=self.my_fbo.texture)
 
        # Use Stencil instruction on Fbo
        with self.my_fbo:
            StencilPush()
 
            Rectangle(pos=(100, 100), size=(100, 100))
 
            StencilUse()
 
            Color(0, 1, 0)
            Rectangle(pos=self.pos, size=self.size)
 
            StencilPop()
        
 
class TestApp(App):
    def build(self):
        return TestWidget(pos=(0, 0), size=(400, 400))
 
if __name__ == '__main__':
    TestApp().run()
```

This PR was made as a first step to fix #1578. But after enabling stencil buffer on fbo, the shader transition shows only a black screen while it is in progress. My experience in openGL is not enough so far to understand why.

Thanks,
tohin