Fix type inference for MSSSIM of Color3 input

[johnnychen94]

No description provided.

[codecov]

Codecov Report

Merging #37 (ecd565f) into master (bb4e7d1) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #37   +/-   ##
=======================================
  Coverage   98.72%   98.73%           
=======================================
  Files           5        5           
  Lines         157      158    +1     
=======================================
+ Hits          155      156    +1     
  Misses          2        2           

Impacted Files	Coverage Δ
src/msssim.jl	100.00% <100.00%> (ø)

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