Stop special-casing the disable parameter for setting.

There are more parameters then just 'disable' that should be set before
chain finalization; in particular, DeconvolutionSharpenEffect compiles
the matrix size into the shader. Instead, just set all the parameters
once right after the chain has been built, which includes the disable
parameter.
Let Movit effects supply their own fingerprint.

This allows effects to signal that some sort of change means the chain
needs to be regenerated. In particular, this unbreaks changing the
matrix_size parameter of DeconvolutionSharpenEffect; if you change it,
the entire chain will now be regenerated, instead of getting an assertion
failure.
Correct maximum matrix size for DeconvolutionSharpenEffect.
Give movit_mix's reverse parameter the same metadata as in the core mix transition.
Make movit_mix ignore empty "mix" parameter strings.

Previously, the parameter would have to be entirely unset
(ie., NULL) for the transition progress to be used, which
was suboptimal in some cases.