plugins/screencast: Introduce base ScreenCastSource type

Currently, when screencasting a window, kwin may render a window into a
temporary offscreen texture, copy that offscreen texture to the dma-buf
render target, and discard the offscreen texture.

Allocating and deallocating offscreen textures is inefficient. Another
issue is that the screencast plugin uses Scene::Window::windowTexture().
It's a blocker for killing scene windows.

This change introduces a base ScreenCastSource type. It allows us to
move away from Scene::Window::windowTexture() and make the dma-buf code
path efficient with applications such as Firefox that utilize
sub-surfaces.

With the ScreenCastSource, kwin can also provide screen cast frames with
arbitrary device pixel ratio.