Sprite rendering demo. In this version we're doing the animation computations on the GPU, using transform feedback and a vertex shader, then rendering is performed in 2 passes, with depth testing enabled: 1) Sprites are rendered front-to-back, opaque fragments only, blending is disabled. 2) Sprites are rendered back-to-front, transparent fragments only, blending is enabled. Sorting is free, because we're animating double the amount of sprites rendered, the first batch is sorted f2b, the second is sorted b2f. Ordering is achieved by modifying the z-axis position of the sprites in the vertex shader.