* Moved `UIComponent::GetViewport()` to `Scene` class.
* Simplified resource initialization of `UIComponent` and removed need of overriding `IsEnabled()`.
* Renamed `UIComponent::GetElement()` to `GetRoot()` to be more consistent with UI subsystem.
* Removed ability to set root UI element to `UIComponent`, instead to add UI elements should be added as children to element renturned by `UIComponent::GetRoot()`.
* Cleaned up debug rendering in HelloGUI sample. Debug data is now rendered after pressing F2.
* Separated batches and vertex buffers of normal UI and UI rendered to texture.
* Cleaned up `Render()` method adding additional `surface` parameter to render into. Passing null renders on to screen.
Support for rendering UI to texture.
This change adds support for rendering GUI into a texture. This is achieved by adding `UIComponent` component to `Node`. `UIComponent` overrides material of `Node`'s `StaticModel`. UI subsystem is changed so that batches belonging to UI elements owned by `UIComponent` are rendered into a texture that is also contained in `UIComponent`. Input is handled by UI subsystem, so anything that worked on normal UI will work on UI rendered on to 3D object. This required a minor change for input handling. Previously "screen" was considered surface of entire window. Now screen is considered area of root_element_position + root_element_size. This is fully backwards-compatible.
There is one issue i am unable to solve: since on OpenGL texture coordinates start from bottom-left corner a workaround is needed. Texture is flipped by projection matrix, however it breaks scissor test. I do not know how to properly fix it therefore scissor test is disabled when rendering into texture on OpenGL. Issue is clearly visible when scrolling a `ListView` in `HelloGUI` sample when UI is rendered on to a cube.
Other changes: lua and angelscript bindings, HelloGUI sample can toggle between rendering on screen or rendering on cube, added a ListView to HelloGUI sample window. `UI::DebugDraw()` support.
