This vertex shader produces the following, when drawn using indices 0..3:
1 | 0-----x.....2
0 | | s | . ´
-1 | x_____x´
-2 | : .´
-3 | 1´
+---------------
-1 0 1 2 3
The axes are clip-space x and y. The region marked s is the visible region.
The digits in the corners of the right-angled triangle are the vertex
indices.
The top-left has UV 0,0, the bottom-left has 0,2, and the top-right has 2,0.
This means that the UV gets interpolated to 1,1 at the bottom-right corner
of the clip-space rectangle that is at 1,-1 in clip space.
