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Text: Reset alignment offsets when passed Alignment::NONE

This is to allow the end-user the ability to clear alignment
offsets. This is necessary when you wish to apply a new text alignment
on an existing text (of which had an alignment set previously).
Introduce getter && setter for current RenderWindow instance

This is to allow us the flexibility to make calls that require
access to the current renderer device, and is similar in concept to
how Rocket::Core::SetRenderInterface and Rocket::Core::
GetRenderInterface is used in the engine.

* Texture: Change ::set_render_target argument from void to
RenderWindow&; this allows us additional flexibility.

* Renderer: Rename ::reset to ::set_render_target.
Gradient: Replace direct SDL calls w/ nom::render_interface call