Pass mutable strings to Chroma

Chroma currently expects all strings to be mutable.

Calling this from frozen_string_literal files works fine, but not
on ruby 2.7 where things were handled differently. This syntax
can be removed when either a) Chroma accepts frozen strings or
b) ruby 2.7 support is dropped

Author: gravitystorm

lib/glug/chroma_extensions.rb

@@ -13,7 +13,7 @@ def mix(other, weight = 50)
       g = (rgb.g * p + other.rgb.g * (1 - p)).round
       b = (rgb.b * p + other.rgb.b * (1 - p)).round
 
-      Chroma.paint("rgb(#{r}, #{g}, #{b})")
+      Chroma.paint(+"rgb(#{r}, #{g}, #{b})")
     end
   end
 end

lib/glug/hsluv_modifiers.rb

@@ -66,7 +66,7 @@ def to_hsluv
 
     def from_hsluv(h, s, l) # rubocop:disable Naming/MethodParameterName
       rgb_values = Hsluv.rgb_prepare(Hsluv.hsluv_to_rgb(h, s, l))
-      Chroma.paint("rgb(#{rgb_values[0]}, #{rgb_values[1]}, #{rgb_values[2]})")
+      Chroma.paint(+"rgb(#{rgb_values[0]}, #{rgb_values[1]}, #{rgb_values[2]})")
     end
   end
 end