One point worth noting here is that the glUniform* calls do not take a
program object handle as a parameter. The reason for this is because the
glUniform* calls always act on the current program that is bound with
glUseProgram. The uniform values themselves will be kept with the program
object. That is, once you set a uniform to a value in a program object,
that value will remain with it even if you make another program active. In
that sense, we can say that uniform values are local to a program object.