Revise ScreenAlignedTriangle and ScreenAlignedQuad (again)

Typically, a screen-aligned quad/triangle is used to do post-processing using shaders; however, the current implementations of ScreenAlignedTriangle and ScreenAlignedQuad do not allow this: they simply display an existing texture. This should be reflected in the class name to avoid confusion.

Further, since their functionality is reduced to displaying an existing texture, they are pretty much identical, so what is the point of having both?

I would suggest to replace the current ScreenAlignedTriangle and ScreenAlignedQuad classes with:
- A (single) `TextureDisplay` (or similar) class that displays an existing texture on screen
- A screen-aligned triangle geometry class (the current `Triangle` is not screen-aligned)
- A screen-aligned quad geometry class (the current `Quad` can be used with size = 2)

The latter might not be required if a screen-aligned triangle is considered state-of-the-art, making the screen-aligned quad obsolete.