Struct Specifiers in Uniforms

Verifies that structure specifiers work with uniforms.

Shaders are of the form:
    precision mediump float;
    uniform struct S { $(type) field;} s;
    void main() {
        // All uniforms are required to be zero initialized. Add the color green
        // to make the rendering test pass.
        gl_FragColor = $(asVec4) + vec4(0.0, 1.0, 0.0, 1.0);
    }