Check if functions need to be called instead of always doing so.

[ianhi]

Problem

Because user functions get called for all kwargs that are registered, including any kwargs that were added as scalars for things like alpha. For example:

def test_imshow_scalars():
    # and by proxy all the scalar handling
    def mask(min_distance):
        return np.random.randn(10, 10)

    fig, ax = plt.subplots()
    iplt.imshow(mask, min_distance=(1, 10), alpha=(0, 1))

changes the array for every change of alpha. I'd expect it to not do this - this is also a detriment to performance.

Proposed Solution

1. Some sort of caching of args and checking them
2. Multiple calls to add_kwargs that deal with the scalar kwargs

Additional context