Color mixing feature (#15347)

# Description

This PR ports the color mixing feature from BambuStudio.
The port is based on the previous work by @ianalexis in #15231.
This PR completes the port and fixes various bugs.

Several improvements were also made during the porting process.

WIP


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src/libslic3r/Preset.cpp

@@ -1185,6 +1185,7 @@ static std::vector<std::string> s_Preset_print_options{
     "flush_into_infill",
     "flush_into_objects",
     "flush_into_support",
+    "enable_mixed_color_sublayer",
     "tree_support_branch_angle",
     "tree_support_angle_slow",
     "tree_support_wall_count",