Test case #2: PS Mem fetch with output

[RenfengLiu]

The test cases add a new knob texture-count, and can be run with

{
    "deterministic": true,
    "enable-skybox": false,
    "enable-spheres": false,
    "enable-metrics": false,
    "fullscreen-quads-count": 1,
    "fullscreen-quads-type": "Texture",
    "fullscreen-quads-single-renderpass": true,
    "vs-alu-instruction-count": 100,
    "texture-count": 2
}