A pixel shader for engo

Pretty straightforward. Creates a pixel shader using the provided FragShader. Play with neat designs and such from places like Shader Toy right inside your engo games! The demo shows off how to use it in engo.

The fragment shader has access to three uniforms. If you think of more, submit a PR or create an issue and I can add it. The uniforms are the following:

uniform vec2 u_resolution;  // Canvas size (width,height)
uniform vec2 u_mouse;       // mouse position in screen pixels
uniform float u_time;       // Time in seconds since load
uniform sampler2D u_tex0;   // Drawable Tex0
uniform sampler2D u_tex1;   // Drawable Tex1
uniform sampler2D u_tex2;   // Drawable Tex2