Implement GL_SCISSOR_TEST at the RenderTarget level to support
arbitrary clipping of rendered areas, and implement translation
functions to support scrolling of arbitrary rendered areas. These
primarily support the implementation of nested GUI elements that
can be visibly clipped within and scrollable by a parent container.

- Add scissor_stack vector, push/popScissorRect() to define and
  access visible areas defined as sp::Rects.
- Add translation_stack vector, push/popTranslation() to define and
  access positional deltas, translate them to pixel coordinates,
  and track the render's total translation value.
- Add getTranslation() to expose the total current translation
  value, to allow for relative positioning to a translated render.
- Add static applyProjectionMatrix() to apply a translation to a
  2D projection matrix, for mapping a virtual coordinate to a
  relative screen position.

This allows the definition of a GUI container that renders and
interacts with only what is within a scissor rect, while allowing
overflow content to be scrolled (translated) into that rect.

For example, given:

    void GuiScrollContainer::drawElements(
        glm::vec2 mouse_position,
	sp::Rect /* parent_rect */,
	sp::RenderTarget& renderer
    )
    {
        sp::Rect content_rect = getContentRect();

        renderer.pushScissorRect(content_rect);
        renderer.pushTranslation({0.0f, -scroll_offset});

        glm::vec2 layout_mouse = mouse_position + glm::vec2{
	    0.0f, scroll_offset
	};

        for (auto it = children.begin(); it != children.end(); )
        {

	    ... pass translated mouse events through to scrolled
	    contents ...

	}

        renderer.popTranslation();
        renderer.popScissorRect();
    }

Rendering of all child elements of this container is clipped to
the parent's content_rect dimensions and shifted vertically by the
translation offset. Mouse events are also passed through to the
translated positions of the child elements.