Add support for runtime constants (specialization)?

[almarklein]

SpirV supports specialization: setting constants at runtime, can be useful?

[axsaucedo]

+1 on this, it would be great if it could be defined similar to the parameters, as:

@python2shader
def compute_shader_multiply(
        index: ("input", "GlobalInvocationId", ivec3),
        data1: ("buffer", 0, Array(f32)),
        data2: ("buffer", 1, Array(f32)),
        data3: ("buffer", 2, Array(f32)),
        constants=[Constant("LEN_VAR", 0, )]):
    i = index.x
    data3[i] = data1[i] * data2[i]