Testcase: Alpha blending

[RenfengLiu]

Added new knob quad_blend_mode with value "none", "alpha", "disable_output". This also replaced the previews knob disable-ps-output, which now can be set as "quad_blend_mode":"disable_output"

{
    "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,
    "disable-ps-output": false,
    "fullscreen-quads-texture-path": "benchmarks/textures/tiger_2364x2880.jpg",
    "viewport_height_scale": "1",
    "viewport_width_scale": "1",
    "quad_blend_mode": "alpha"
}