build(scripts/conv): add script to gen value convs

The script generates tables for converting between
different ranges of color values.

scripts/conversion-tables.tcl

@@ -0,0 +1,30 @@
+#! /usr/bin/env tclsh
+
+package require textutil
+
+set varDeclTemplate {static const uint8_t %s[] = {%s};}
+
+proc table {from to} {
+    set ratio [expr { $to * 1.0 / $from }]
+
+    for {set i 0} {$i < $from} {incr i} {
+        lappend table [expr { int($i * 1.0 / $from * ($to + $ratio) ) }]
+    }
+
+    return $table
+}
+
+proc format-table {name table} {
+    set lines [textutil::adjust [join $table {, }]]
+    set indented [join [split $lines \n] "\n    "]
+
+    return [format $::varDeclTemplate $name "\n    $indented\n"]
+}
+
+foreach {from to} {256 32 256 64 32 256 64 256} {
+    dict set tables hicolor_${from}_to_$to [table $from $to]
+}
+
+puts [join [lmap {key value} $tables {
+    format-table $key $value
+}] \n\n]