Get a Uniform for this shader.
uniform = Shader:getUniform(name)
- The name of the uniform.
- The uniform. If the uniform doesn't exist or there is an error, this will be nil.
It is good practice to save the result of this function, as calling it is more expensive than using an existing result.
If the Shader object is garbage collected (falls out of scope, or you lose the reference to it), the uniform object will no longer be valid.