Used abstract type AbstractMatrix in encoding

[ymtoo]

The PR fixes the error:

julia> x = Float16.(channelview(rand(RGB{N0f8}, 180, 320)))

julia> WebP.encode(colorview(RGB, N0f8.(x)))
ERROR: MethodError: no method matching encode(::Base.ReinterpretArray{RGB{N0f8}, 2, N0f8, Array{N0f8, 3}, true})

Closest candidates are:
  encode(::Matrix{TColor}; quality, transpose) where TColor<:Colorant
   @ WebP ~/.julia/packages/WebP/G4Y4q/src/encoding.jl:1

Stacktrace:
 [1] top-level scope
   @ REPL[34]:1

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 23.59%. Comparing base (682e7f0) to head (d9ff8fe).

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Additional details and impacted files

@@            Coverage Diff             @@
##           master       #4      +/-   ##
==========================================
+ Coverage   19.49%   23.59%   +4.10%     
==========================================
  Files           4        5       +1     
  Lines         354      373      +19     
==========================================
+ Hits           69       88      +19     
  Misses        285      285              

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[stemann]

Thank you! Just in time for v0.1.0 🚀