Add default fragment shader to selection shaders

[TaylorMutch]

Since we are actually drawing colors per object we want to test against (rather than just perform depth tests), we need to add a fragment shader.

[nikmolnar]

Why did this work previously?

[TaylorMutch]

This previously worked because in the vertex shader, we specified a single output color per vertex:

// vertex.glsl
uniform vec4 color;
out vec4 finalColor;
void main() {
    finalColor = color;
}

Technically this is correct, but we're telling the shader program that each vertex needs to be assigned a color during vertex assembly. This way, with specifying the color as a uniform color in the fragment shader, there is no need to add an additional input variable to the fragment shader. We just assign gl_FragColor (or technically, the single out vec4 from the fragment shader) to be the uniform we want for the whole shape.